Our old cat aka evil cat has bony growths on her spine (can't spell the condition) and medacam made a huge difference. She also dropped a 'nugget' which was pretty unpleasant. Nothing since the metacam. Regrettably it hasn't reduced her evil, but then, she has been working on that for eighteen years and isn't giving it up now. Metacam may make all the difference for your old friend.
She has failing kidneys, is almost certainly mainly blind and is extremely creaky. The vet has been muttering about thyroid etc as there is more meat on a butcher's pencil but can't take blood samples as evil cat is too violent (yes, really, last time she was at the vets it took three of them to take a urine sample, now the vet visits us, next door helps out wearing welding gloves and evil cat still always manages to draw blood).
The thing is, I wouldn't pts for blindness, creakiness, grumpiness etc. Evil cat is still getting a good quality of life, snoozing in the sun, generally bullying me and eating twice her bodyweight in expensive food. She is also allowed to steal a lot more than she used to and she uses this fact shamelessly.
I would consider pts if she was generally and consistently incontinent and the vet could not suggest appropriate treatment. It can be incredibly distressing to cats, and if it was something that couldn't be easily dealt with, at eighteen, then I would think it has to be considered an option. Failure to groom can also be a source of distress to a cat. Cats are not human. They do not process things like us. Being blind is not the same for a cat as it is for a human, and evil cat is adjusting well. Incontinence is a bigger problem for a cat than a human.
Treatment for arthritis may make all the difference and you get your old friend back. While your old cat has any quality of life then yes you should keep trying. However I do think that you should consider pts as an option if your cat appears to be suffering. Talk frankly to the vet.