Kokomo was not only the last hurrah for the BB, but also the last hurrah for the BB's contemporaries' shot at getting their new music played on youth-oriented Top 40 radio and thus becoming a hit. If there was a vintage act that had a hit after Kokomo, it could not have been that much later than 1988 because by the early 90's, radio was ignoring thee new records of the BB, McCartney, Stones, etc. Clapton being the sole exception to this.
As for Kokomo, I liked it in high school, disliked it in my early 20's and have mellowed toward the moderate in my 30's. What no one has mentioned yet is how much Mike, Terry and VDP borrowed from cousin Brian to complete the song. The solo vocal in the chorus sung by Carl is the kind of soaring high vocal young Brian might have used. Plus the chorus hook has the same type of key changes Brian was fond of using (see California Girls, Little Girl I Once Knew, H & V). Looking back, I'm surprised Landy never had Brian try and sue over the song because beneath the Jimmy Buffett vibe, the Brianisms are quite obvious.
Cher had a big hit in the last 10 years. I don't recall exactly what year, but it sucked just like most top 40 hits in America at the time.
I am not sure that you can sue someone for being influence by them. What if Chuck Berry sued for Surfin USA? (Oops he did!) Maybe Fun, Fun, Fun would be a better example. Or Phil Spector for Don't Worry Baby.
Personally, I feel that there are plenty of BB songs post 1996 that make me cringe. Kokomo is not one of them. Not a classic, but a nice tune.