You might as well ask what would have happened if Brian Wilson was not deeply mentally ill and what would have happened if Mike Love was not completely excited about hitting the oldies circuit and making a great living singing the same 20 songs for the next 50 years?
If you’ve ever read an interview with Mike Love, you know that he inspired part of “Back in the USSR,” because he tells that story in EVERY interview. You also know that his happiest points in the band where the early 80s during their “America’s Band” hokey nostalgia period where they were playing to huge crowds with him in complete control through “Kokomo” because he can now say that he wrote a number song without Brian, and Brian never wrote a number one song without him.
The Beach Boys story was never going to end harmoniously (that’s a double pun, folks).
“Kokomo” was primarily written by John Philips, the creative force and songwriter of the Mamas/Papas.
IIRC, Scott McKenzie contributed to the song, too.