It's very personal, so, please don't let anyone persuade you to do something that doesn't feel right.
But, we would probably just let a 15yo who is otherwise happy enjoy the time she has left. It's hard- surgeries are risky on seniors (though, the recent surgery going well is encouraging), and obviously, there's pain with recovery. Would a dog choose to go through the pain for the chance of a few more years? It's hard to imagine *our* dogs choosing that, but, I can absolutely understand someone feeling very differently about it/with another dog.
I'm sorry you're dealing with this decision. Taking into consideration how much the vet upsets your dog, the fact that she doesn't seem bothered by the bump, and her age, I would choose not to have the surgery done. At this point I wouldn't want to put her through the stress of surgery and then a painful recovery.
Like cmeon and star gaze said I would tend to lean towards leaving it unless it needed to be removed. That is my personal opinion and by no means should you do what I say. Do what you think is best for your pup.
Our heeler is 14 and he is getting the fatty tumors, one is right by his penis. The Dr. recommended we not pursue surgery unless it starts to strangle or grow significantly (because it might press on his bladder). He hobbles around happy as he can be and we don't want to put him through the stress of a surgery (the canine removal ruined the vet for him).