I think that most people if they had a dog that was suffering for whatever reason, they would even begrudgingly have it pts. It is only humans that have to suffer life for life's sake. If that wish is freely given, which it appears to have been hear, and for a valid reason, who are we to judge.
I only worry in the case of the elderly, especially here where the estate inherits the debts, that some might be shuffled off before the care home bill cancels out the inheritance entirely. That granny might feel impelled to go for the injection and leave something to the family, rather than hang on in there.
Pointy - I am NOT talking about your gran here. Just musing, that if I was 80 odd and failing, and had the choice in paying care home fees vs giving my dd/gc something for the future and have them not watch me degenerate - weil I might choose that option too.