I do believe Lennon was/is a hero to many, FBOFW, and did affect the lives of many who admire/admired him.
THe saddest thing to me about his death was that he appeared to be entering a new phase of his public life with some new perspectives perhaps as a result of being a newly engaged father and family man. We'll never know what kind of new musical directions he might have gone down. As one of the undeniable great pop/rock musical talents of his time, and not one to rest on is laurels I would say, he might well had delivered a lot of interesting new music and remained relevant for many years IMO.
I believe pop/rock music would be much different today had he lived into the 1980s. No way to know for sure though.