I have no opinion of sustainability, but one can look at the terms and see what you're getting. The license is here, theoretically, but keep the piece of paper in your box, if you have a piece of paper, just in case the online version disappears / changes. Part 2(a) says:
The software is licensed, not sold. Under this agreement, we grant you
the right to install and run one instance of the software on your
device (the licensed device), for use by one person at a time, so long
as you comply with all the terms of this agreement.
The expression "on your device (the licensed device)" could easily make one think that it is tied to a specific machine, but that is only true in the sense that it is for a single machine (at a time). Regarding changing machines, part 4(b) would be relevant (this assumes you didn't acquire the software in Germany, where there is special law):
If you acquired the software as stand-alone software (and also if you
upgraded from software you acquired as stand-alone software), you may
transfer the software to another device that belongs to you. You may
also transfer the software to a device owned by someone else if (i)
you are the first licensed user of the software and (ii) the new user
agrees to the terms of this agreement. You may use the backup copy we
allow you to make or the media that the software came on to transfer
the software. Every time you transfer the software to a new device,
you must remove the software from the prior device. You may not
transfer the software to share licenses between devices.
So you can continuously transfer from machine to machine, as long as you uninstall the OS from machines that you are done with -- you cannot multiply copies by serially upgrading hardware. If you only need 1 machine, you're set for life (your life, or the person why acquires it from you, but no further than that). Also please note that the claim that Windows 10 is the last version is not a binding license term, so legally speaking, W10 could go the way of XP.