If we talk about positions then all of these players have at least one substitute, if not several ones apart from Mahrez. We know also that Mahrez didn't participate a lot in the previous season and was less effective comparing to two or three years ago.
Obviously having many players leaving the club will destabilise it but Manchester City know already that a transition phase will occur especially after they won everything especially Champions League title
Even so, Manchester City will not immediately become weak next season. Because Manchester City are very prepared for the transition phase they are currently experiencing, so there is still a possibility that Manchester City will continue to be strong next season, even though there might be a difference from last season when they won the championship. And in terms of substitute players, indeed it must still be there in order to anticipate unexpected things like you said that.Obviously having many players leaving the club will destabilise it but Manchester City know already that a transition phase will occur especially after they won everything especially Champions League title
I'm sure it won't be a problem if for example Mahrez really leaves in this transfer window, because Mahrez is also only focused on playing on the right wing. Therefore, of course Pep Guardiola will also adapt it to Manchester City gameplay which will be used later if, for example, Mahrez realy leaves. After all, at this moment also Manchester City has succeeded in signing in Kovacic and it seems that Manchester City are exploring the midfield for next season to be better after losing Gundogan. But regarding rumors of Mahrez departure, i personally still not entirely sure if Mahrez leaves at this moment, because Mahrez also is still under contract.