
Yes I've had plenty of experience of this although mainly in cats -I breed Persians & Exotics which have large round eyes so they are prone to it. The big problem is, an ulcer that doesn't heal CAN cause the eye to rupture, especially if it gets infected -I have had this happen no less than 3 times and it is NOT nice. :( The first cat lost his eye as it literally burst open, he's happy with just the one now. The second had the patches done as there was just a hole in the eye and the entire eyeball wasn't ruined. That worked really well. So although your dog is old I would worry about ruptures if he doesn't have the surgery.
Usually if an ulcer doesn't heal easily, the first step of action is to stich the inner eyelids shut to allow the eye a chance to heal without being disturbed.
I think you should ask your vet again about the risks just to play safe. When my Cavalier had an eye ulcer it healed up within a week; had it not done so my vet said the eye would need to be stitched shut.
Good luck!!
Marianne PS I think the added complication of diabetes actually makes the eyes more likely to ulcerate (and harder to heal) AND to rupture as often there is extra pressure in the eyes of diabetics.