The part Lennon wrote is what made the song such a great one...
What ??? McCartney gave it a structure that paved the way for all the great prog rock bands of the 70's. Without him, that song would have been a mere psychedelic song like there had been hundreds before its release.
Lennon's post Beatles career was much better. What can you saw about "Stand By Me", "Jealous Guy", "Instant Karma", "Watching The Wheels"
Jealous Guy is the one I really like.
Instant Kharma is OK.
Stand By Me, Allpro said it all.
I don't know the other one.
But none compare to Band on the Run by the Wings. A song that has a structure in it. Again, it sounds like prog.