I'll answer out of order for a more logical presentation.
Why would lawyers "leave the practice altogether", "[b]efore these
lawyers lower their prices further they will"?
Only about 30% of people earn a four year college degree. A lawyer is a college graduate who had to be in roughly the top 50% of his or her undergraduate class to be admitted to law school, so that puts him or her in the top 15% of academic ability. To actually finish law school and pass the bar exam, a lawyer has to be in roughly the top 12% of academic ability compared to the general public.
Lawyers have strong reading and writing skills, reasonable communication skills, understand complex business and family matters, and are reasonably well organized compared to members of the general population. They also come with background checks completed in advance to weed out risks of criminal or otherwise dishonest or high risk employees. These are highly transferrable skill sets and a J.D. is a good substitute for an M.B.A. in many positions.
Newly minted lawyers who choose to work in business management or a kindred field can typically secure middle management jobs in business and finance without much difficulty assuming that they weren't disbarred or had their licenses suspended for dishonorable reasons, or in a field related to their undergraduate major in college.
So, if a lawyer can't earn a minimum compensation as a lawyer that rivals what the lawyer could earn in middle management, the lawyer will leave law and work in a non-legal post.
Many lawyers do just that. And, indeed, since grant based financial aid is very rare for law school, most newly minted lawyers who aren't independently wealthy have huge student loan debts ($200,000 at the completion of an undergraduate degree and law school wouldn't be unusual), so if they want to avoid a default or an extremely long repayment period for a debt that can't be discharged in bankruptcy, they aren't free to accept a very low income even if they are willing to live modestly.
The floor of what a lawyer can make in terms of compensation before having to seek better paying non-legal work is on the order of $40,000 to $60,000 of net compensation before taxes per year depending upon the local cost of living.
In the long run, average legal incomes need to be even higher, because if too many people end up earning only marginal compensation, more potential lawyers will choose other career paths (e.g. as loan officers at banks, in insurance sales, in real estate brokerage work, as small business owners, as middle managers in big businesses, as H.R. officers, as financial analysts, as management consultants, etc.) which provide greater or similar returns with less student loan debt, fewer years of education, and less risk.
Why can't fees be lowered "enough on individualized, bespoke services
to meet the unmet need at an aﬀordable price"?
Lawyers have bills to pay - a fully furnished and equipped office, at least a part-time per lawyer staff person, suits and dry cleaning, bar dues, and compensation for themselves (the floor on which was discussed above).
But, providing "individualized, bespoke services", rather than mass producing services with a paralegal heavy practice (the way most debtor oriented consumer bankruptcy shops run, and the way prosecutor's offices and public defender's offices run), then it takes a lot of hours to provide those services. (The average prosecutors office has expenses of about $50 per felony case or less, not including law enforcement efforts to gather evidence and put the facts together coherently, and public defenders have even lower expenses per average felony case, perhaps $35 or less.)
Even the most basic legal services (e.g. writing a simple will, defending a simple residential eviction or collection case or a DUI) takes 4-8 hours of lawyer time to accomplish, and you can't spend 100% of your time providing legal services either, as you have to devote some time to marketing, evaluating potential clients, supervising bookkeeping and administrative tasks, continuing education, etc.
So, at most, a lawyer can handle at most about 200 very simple cases a year, or fewer cases that have any sophistication, and sometimes people don't pay their bills or don't pay on time. A 10%-25% bad debt rate would be pretty typical in small firm practice.
Realistically, each lawyer needs about $90,000+ of gross collected revenues to cover expenses and make a floor compensation amount. So, a lawyer has to collect at least $500 to make a very modest living and just barely get by on a typical very small case provided at an extreme discount. A lawyer needs more like double that amount of gross revenues per very small case to make a decent living, and that all assumes that marketing efforts can reach enough people to keep the lawyer busy. If a lawyer's marketing or AR collection rates fall, fee collections per case have to rise.
The trouble is that lots of people can't afford to pay a lawyer $500 to $1,000. This isn't affordable to them. Keep in mind that 40% of Americans don't have the ability to pay an unexpected $400 expense, and that the percentage is much higher in a place like Alabama or West Virginia. A $500-$1000 legal fee expense for even a very simple matter is something that most people with "unmet legal needs" simply cannot afford to pay no matter how badly they need those services.
These are the people with "unmet legal needs", and the cases they face are often cases like defenses of criminal charges likely to result in some sort of conviction, or no asset bankruptcies, or defenses of foreclosure or eviction or collection cases, non-employment related immigration cases, or child custody cases, or defense child support collection actions, in which case there is no significant pot of money from which fees can be paid, even if a lawyer's client wins.
(In cases where there is a significant potential "pot of money" like money owed for work done or a personal injury case, legal needs usually are met.)
Also, while lots of Americans can't afford even a $400 legal bill, there is a pretty significant group of businesses and upper middle class and wealthy individuals who can afford to pay tens of thousands or hundreds of thousands of dollars per legal matter, and for whom a lawyer can provide value added to the client even at that price.
For example, businesses and landlords that need commercial leases to be negotiated, documentation of and negotiation of multi-million dollar business financing deals, tax planning for a complex conglomerate, personal injury litigation in cases with clear liability and serious injuries, business disputes, representation of clients in connection with the bankruptcies of medium and large businesses, employment litigation, white collar crime defense, etc.
This kind of work requires a lawyer's marketing effort to be much more modest and dramatically reduces the time spent evaluating new cases. A lawyer may need only five or ten cases like that per year instead of 200 small potatoes cases, while earning much better compensation.
So, the legal needs of those who can afford to pay are met, and the legal needs of people who can't afford to pay lawyers at any fee that a lawyer can break even at (realistically closer to $125-$175 per billable hour after bad debt and time generating work and idle time until new clients are secured is considered), and who don't have economic circumstances from which legal fees can be extracted in their cases, go unmet.
Until such time as legal work can be provided with fewer attorney hours, or by someone who has less well paying alternatives to practicing law, and with less expensive educations, it will remain this way indefinitely without some sort of subsidy or insurance coverage.
Also, keep in mind that practicing law provides an irregular income in small firm practice due to irregular payment timing and ebbs and flows of new clients, so to keep to any reasonable set of fixed living and overhead costs paid, a lawyer must have months that generate much more than the lawyer's "monthly nut" in order to generate savings for lean months.
Finally, a lot of the very low paid lawyers are retirees, part-time employees who are also homemakers, are people moonlighting as lawyers in addition to another job, or are charitably inclined independently wealthy people. So, they have additional sources of income and aren't supporting themselves from full time work as lawyers. These are the only people who can afford to work as lawyers for lower amounts of income per year.
But, even then, independently wealthy people normally have the best access through social and family networks to people who have an ability to pay them to work on high dollar matters, so they normally end up in high paying jobs where their "rain making" capacity is part of why they are paid so well, rather than giving their services away on a charitable basis.
An important corollary of this analysis is that the problem of unmet legal need cannot be solved simply by producing more lawyers, because if you produce more lawyers and all legal need capable of paying for the work is met, the new lawyers will simply pursue non-legal careers.