The only cat we had to have euthanized was one with a stomach tumor. We kept going with him until he really seemed too miserable to keep going. He was vomiting blood, but that wasn't the reason we chose to end things-- it was the awful misery he was in while he was doing it. And the fact that he wasn't able to eat. If he'd been a human, I'd have tried to give him the option to make his own exit (not legal in many places, I know). We couldn't have that conversation, so I had to make the best decision for him that I could.
I miss him still.
Yvan, I appreciate your point of view. I think our cats are very happy with us, but I know what you mean. Fortunately, though they occasionally miss the litterbox, mostly all we have to clean up around the place is barf. (One is a bit bulimic, and the other has hairballs.) I don't know what I would have done in your situation. My sister in law has a cat that urinates all over the house, and I don't think I'd be able to tolerate that. But confining the cat only to easy-to-clean areas would be hard, too, for the people and the cat.