You're the one who did not acknowledge my point that a devoted spouse's willingness to care for an incapacitated spouse may change over the years. I thought that was a crucial point. You ignored it. And, I still think it is a crucial point, that a spouse's desires for a new spouse may alter the situation and necessitate a change of legal guardianship, while a parents' desire to care for their incapacitated adult child may be less likely to change, since they are not trading in this child for a new child.
Have you considered that if you pass this law, you are putting a Divorce of Damocles over every marriage? Where a woman has a choice of carrying out her husband's wishes (at risk of her in-laws fighting her) or not...which would she be inclined to do? Your law points the incentives away from respecting a patient's wishes, not toward it. Bad laws with incorrectly oriented incentives lead to bad outcomes and unexpected consequences...though in this case, I can take a stab at the consequences and they aren't pretty.
So...how about children, possibly wishing for inheritance? Do you allow them to send in the divorce stormtroopers?
"You're the one who did not acknowledge my point that a devoted spouse's willingness to care for an incapacitated spouse may change over the years. I thought that was a crucial point."
I believe that the Schindlers had her in their home about 10 years ago for a little while and found they could not handle the amount of care she needed - so they sent her back. I don't believe they have gotten any younger since then.