I was in this exact situation a couple of weeks ago.

My cat was a former stray, FIV+. When I got him his estimated age was 3. I had him for 11 years. He started losing weight, having mobility issues, eye problems and dementia. I had to weigh the cost of testing what exactly was wrong and if it was able to be treated versus his quality of life.

There was an added complication in that I myself have terminal cancer and am going through yet another round of chemo so have a lot of medical bills and physical issues with the likelihood of having to make quality of life decisions myself sometime soon.

I had him put to sleep and donated all his supplies to a local feline rescue/no kill shelter to help out other kitties.

I will not be getting another cat due to my tenuous lifespan. I don't want to get a kitty that will need to be rehomed when I pass. I spend time with family member's and friend's pets as I'm able now.

I admit I did do some second guessing of my decision to have him put to sleep, but it was the right thing to do for him.