He would have celebrated his eightieth birthday in October. I wonder how he would have spent those years had he lived ? He, Yoko and Sean were already moving towards a life of simple domesticity in New York and there was already about a six year gap between his new albums (Walls and Bridges in 1974 then Double Fantasy in 1980). I think he would have continued a relatively low profile life, making new music occasionally and perhaps speaking up for the Peace movement he'd championed several years before his death.
That's just my twopenn'orth anyway. We will never know for sure.