I can't see any way of guaranteeing a good new home for a cat or cats if their owner dies or cannot look after them any more - surely all the rescues, such as CP and Cinnamon Trust, can only rehome if they can find someone to take them or at the very least a fosterer - I know in my neck of the woods that would be a tall order, as CP only have three pens in two very large counties
If a decent sum of money is left for a new owner I can't see either who could police the arrangement - my family executors live too far away to be able to keep in contact with anyone who did come forward
I think personally the only realistic way forward as I get older is to limit myself to only one cat, and when she is gone to only take on another oldie, so at least if I'm no longer around there would only be one cat to try to rehome, and if the worst came to the worst only one oldie to pts
another possibility is to have one pedigree cat, which would be easier to rehome, and many breeeds have rescue arrangements themselves