"Imagine" was written in 1970/71 at the peak of his political, drug-laden, Yoko Ono controlled, insanity. That song, and its One-World peace/love hippie sentiment, is far too easy a target for criticism (I don't particularly like it either), and it is not representative of where Lennon was headed a decade later. Reagan or not, Lennon would have produced outstanding rock-and-roll and R&B tunes -- those were what he had in his musical soul.
I was a Beatles fan in the 60's, but I lost track of Lennon from about 1970 until "Double Fantasy" was announced. That album was a disappointment -- I didn't much care for the continuing influence of Yoko Ono, or the personal/romantic nature of the release, but I was glad he was writing and recording again. I don't think for a moment that "Imagine" was "an example of what else was to come". I looked forward to him returning to rock-n-roll and R&B and enriching the world with more great songs.
Mark Chapman stole that from the world, for which he can rot in hell.