All dog lovers want to do what's best for their dog. As difficult as it is for you (us), at some point you (we) have to let them go.
I can't answer your question, but you'll do what's right.

EDIT: As I re-read that this morning, I realize that it sounds quite cold and unsympathetic. I certainly didn't intend it that way. I'm sure you love your dog(s) as much as I love mine. Figuring out what's best for the dog, especially a senior dog is never easy...