We had two ragdoll cats and a cocker spaniel to insure them all each would cost a fortune so we took the decision to just put the money away each month. So far one cat is 14 the dog is 7 and the other cat passed away aged 15 total vets bills over the combined 36 years was just short of £1000 so at present I'd say we ahead. This only works if you have the money put away, the thought of any of them falling ill and me not having the money for a vet!
One abutment is at your dogs age now is when you are likely to get problems but as you say would it just be the right time and decent thing to do at that point to just say good bye.
This is one of those things only you can decide, me if I'd had insurance up to now and I could still afford it then I'd carry it on.