should this cat be put to sleep?

please tell me the truth
several people, some of them vets, have suggested that this cat be put to sleep- one said that he is suffering, others just don't think he has much to live for.

he is approx 13 yrs old, he is blind, and he is almost totally deaf (it is hard to tell for sure, as he has learned how to tell where things are by air currents and vibrations so well, sometimes it seems that he can hear, but tests of his hearing show no response)

he is a diabetic on three shots of insulin a day (most cats do well on two shots, he needs three)

he is in very early renal failure, no treatment needed just a special diet.

he moves very very slowly- you probably can not even imagine a cat moving as slowly as he does, take your average turtle, and reduce that speed! and that is about how fast he goes.
once in awhile he will go faster but he seldom walks at normal cat speed and he does not run or jump- well, rarely. when he was in pain, he did move quickly to get under the bed to hide (the pain is gone now- this was a temporary thing)
he can jump up onto a mattress that is on the floor, but not up on to a regular bed however he has learned how to climb into my bed using a step.

he rarely plays as most cats play is based on sight, but i have been able to get him to use a feather on a string toy sometimes, and occasionally he will attack a catnip toy.

he loves to go outside for walks- so i take him out as often as i can weather permitting.

otherwise, he sleeps a lot, eats about 3 times a day and likes to be pet and brushed but really doesn't do a whole lot.
he will wander from one room to anohter, sniffing things.

so i think that is why i have been told he should be put to sleep, because he doesn't have a lot that he can do.

i don't know how to respond to that- i have a cat who is almost 17 and while she does play occasionally and can still jump (but not as high and nowhere near as often as before) and she spends most of her time asleep, too

so should a 13yr old be more active than he is? do you think that his existance is boring/unsatisfying or i don't know, somehow horrible, for him?

i only had one cat before and she barely made it to age 9- she had been a stray and was just never very healthy

these guys look a lot better than she ever did!
so i just dont' know how to judge

am i keeping him alive for me, or do you think he has a life he enjoys???

how do you know these things???

torn in two about this
althea