There is always a grey zone between being 100% mentally operational and the beginning of, say, age-related dementia.
Before a physician can declare that someone is not able to make clear, rational decisions anymore, there is always a more subtle decline that affects e.g. one's ability to manage money and properties, and often the decision to place someone's finances under supervision/trusteeship requires their mental state to be quite degraded. Before that happens, a senior is sometimes susceptible to suggestion by unscrupulous people and/or family members, or may simply make gross mistakes.
Is there any process, in any country, through which one can make a decision that they cannot undo later?
A (theoretical!) example: if I decided to disinherit my child because they've been really, really awful, is there any way to protect myself from, say, them convincing me to undo my decision when I'm older and emotionally more vulnerable (especially if there's nobody else to control whether I am being influenced by other people)?
Another example with an imaginary horrible heir: could I specify, while I'm mentally OK, that I want to spend my money going into private retirement home XXX rather than public retirement home YYY, even if it is more pricey and my child tries to "force me" to go to YYY because they would inherit more money this way?
I'm curious to know if there is such a thing as a "non-modifiable will".