In any employment, the employer has the right to fire his/her employees depending on given circumstances. In the case of Brooks Wheelan, the Saturday Night Live saw that the comedian was not delivering as he should and not because of a decrease in the number of people watching the show.
The firing of Brooks Wheelan is not an indication that Saturday Night Live is trying to retain loyal viewers by making cast changes. Maybe SNL had another reason for firing Wheelan. Or, perhaps, just perhaps... he simply wasn't funny enough. It's hard to know exactly why they fired him since they aren't commenting on it but whatever reason they had, it's their show and they can fire who they want.
When it comes to something like Saturday Night Live, there's not really any such thing as too many cast changes. It's not like a scripted drama, where the storyline depends on particular characters - the more they can keep things moving and fresh, the better. Changing the cast might also give them a chance to attract viewers who didn't much care for the originals.
"Saturday Night Live" already struggles to appeal to younger audiences who haven't connected with the talent on the show. When it is clear that a particular actor is not keeping fans engaged, it is best to cut him loose and find someone else. Viewers won't stay tuned for a show when they don't like the actors.