I am not sure tha he had a lot more masterpieces left in him. When he became a nicer man, he became a less interesting songwriter. He did not actually deserve to be shot for "Double Phantasy", but it was not a promising pointer to the future.
That said, I miss his voice and I was as stunned and horrified as anyone else on that awful day in 1980. My life would certainly have been poorer without his work.