What an amazing love story, neither of you having never been married until about age 40 then finding a sole mate shows there is always hope. It's the depth of your love that makes this so hard for you, and all of us care and share your loss.
33 years ago a friend of mine died suddenly from a blood clot after kidney stone surgery. It was two years after gradation from high school, she was a wonderful person the pietist girl in the class and otherwise in perfect heath. As far as I know, she never had a first love or the first time.
You have every write to feel all the things you do, you should and need to feel them. I just want you to know that John probably was a good candidate for the surgery. If he was not walking much because of his painful knee he was at risk of dyeing from a blood clot, what is called deep vein thrombosis. Would you feel differently if he had the transplant and died from that surgery? Neither surgery was needed to keep him alive another month or year. Both would only make his life better and might make it longer. When things go well the doctors seem to be better than they are, when they go bad the doctors are easy to blame.
The things the Dr. said after were intended to give you a resin from his death. There may have been better things to say but no one wants to hear: If the insurance... or We could not tell... there are many things that could have made a difference but nothing is risk free and all anyone can do is what looks to be the best at the time.
No mater how much time we have it is never enough. Morn your loss and celebrate what you've had. Remember, we're all in this together and well be here for you.