Yea, why not? Manchester city will still remain strong, or have you forgotten that Manchester city was already strong before he arrives at the club?
Though they aren't as dominant as the are now in the past four seasons but they were very very good, because when Mancini and Pellegrini was the manager of Manchester city, they both won the league and Pellegrini did it convincingly and in an emphatic fashion, so I believe that regardless of if he stays or he goes, Manchester city will still be a dominant force in the English premier league because of the star player at the club.
I agree with you that so far (before Guardiola came) Man City has been a team that has a very good performance and has always been a popular team in the premier league. It's just that Guardiola added the UCL trophy at that time making people think that losing the coach makes Man City weak. In fact, all of that does not only depend on the coach but if a team has quality players, it will certainly be easy to maintain performance and only needs an experienced replacement coach to be able to stabilize the performance.