I had to put to sleep my 14 year old cat two weeks ago. She was diagnosed almost 2 years ago with hyperthyroidism and inflammatory bowel disease, and had breast cancer a year ago (just after I pts my other cat for cancer, timing eh). The cancer was removed but her jaw started deteriorating at the beginning of this year. She had very little jaw bone left and lost the last of her teeth. Most had already been removed approx 3 years ago. She had been on two tablets a day for the whole time. When her bowel would flair up from the simplest of things she was on anti-inflammatories which was another tablet or yukky tasting liquid. Plenty of antibiotics of various types to help after surgery each time to help find one that gets the job done but doesn't aggravate her bowel too much.
She hardly gained any weight back at the start of the year despite having a huge appetite. She had a flareup of her bowel a week before I put her to sleep. Ended up turning into severe diarrhea resulting in dehydration, stressed kidneys and weight loss just overnight. She had even more medication to take and after 3 days at the vets she came home. Sadly my vet doesn't tell me whats happening but it was clear she wasn't going to get better. She had an appetite but couldn't gain weight, she started refusing tablets and her jaw bone ended up deteriorating enough she broke her jaw the day before she was pts.
In hindsight I would have make her comfortable after finding out her jawbone had deteriorated at the start of the year (vet didn't know until after surgery was completed) and put her to sleep as soon as her bowel flared up again before the diarrhea instead of waiting to see if it was going to clear up.
My tabby boy, who was pts 10 months ago due to cancer, hated medication, hated syringe feeding and hated feeling vulnerable all the time. Again in hindsight I wouldn't have put him through the chemo and would have made him comfortable after finding out he had secondary cancer in two places. The trouble was the vet was telling me all of the different treatments, talking about specialists etc. Far too much false hope for both cats (Ill probably be finding a vet who actually tells me whats going on and will advise when to pts in future).
Personally after these two experiences im more of the mind of looking at whats wrong now and what will happen when they reach peak illness from whats wrong in the near future. If they have cancer whats going to happen when the cancer gets too much for them, whats going to happen when the bowel disease gets too severe, what happens when the jaw bone deteriorates more. Hoping they might find some happiness in their life while deteriorating isn't going to make the suffering worth it.