Some of those Fox stations kept Arsenio when Chevy Chase appeared, but they pushed Arsenio after Chevy Chase and didn't move Arsenio back when Chevy Chase was cancelled
In Philadelphia, WTXF moved Arsenio to 11, and aired Chevy Chase at midnight. I believe at the time WTXF was owned by Paramount, although I'm led to believe that even if Fox owned the station at the time, it might not have cleared Chevy there at 11 right away either, instead choosing the syndicated show with a proven audience.
If recall correctly, Chase's show didn't last long at all, and his impact to Arsenio was probably nominal. Arsenio's show may have been troubled on its own.
Did 'Seinfeld' and 'The Simpsons' reruns hit syndication around that time? Stations might have preferred airing them instead of talk, already covered with CBS and NBC.