He was definitely someone they have used a lot and he is still young enough to provide a few more seasons without a doubt, but if they get a great offer than nobody is unreplaceable for them. If you offer a lot, I bet that they would be willing to sell even De Bruyne or Haaland as well. It all depends on money for them and they are trying to build a team that is a full team and no single player makes the difference.
As we all know De Bruyne and Haaland does, and the yare amazing, but even if they sell one of them for a ton of money, and in order to get them you need to offer record numbers, then they would be able to replace them with another player and still be a good team, they will not be bad just because they lost one player.
Manchester city is in an unstable condition in this transfer window because they have to make a big effort in keeping their players from leaving in this transfer window. Previously, two of their core players had been recruited by other teams in this transfer window, such as Gundogan who was recruited by Barcelona and Mahrez who was bought by Al Ahli. So recently, news has emerged that Bernardo Silva has started to be linked with a number of big teams in Europe and that Kyle Walker has also been eyed by a big Bundesliga team.
I think Manchester city should be able to hold Bernardo Silva and Kyle Walker for next season because they had a big influence on Manchester City game last season. So Pep Guardiola has to watch out for that if he doesn't want to lose a lot of their star players in this season's transfer window.