I think he would be absolutely appalled at the state of the world today. Many people I've talked to who lived through the 1960s claim that today seems much worse, and that much more is at stake today than then. I don't know whether to agree or not, I didn't live through the 1960s. But hearing a Vietnam Vet tell you that "today is much worse" sends electricity down your spine. Maybe Lennon would've agreed? Who knows.
Lennon would have said something, but his influence would be greatly diminished. Musicians don't have the pull and/or freedom that they used to, nor are they taken as seriously as they used to be (judging from what I've read from the past) in the mainstream.
Who knows what would've happened musically. "Double Fantasy" was not selling well, and didn't garnish much critical praise, until after his assasination. Some of his later solo work didn't point in great directions, but comebacks always happen.