If Manchester City sell Bernardo Silva, it will be very risky for them. An experienced coach like Guardiola does not take such risks. Because Manchester City's midfielder, one of the most important players, İlkay Gündoğan has left the team. Now if Manchester City sells Silva too then Manchester City will have to rely solely on Kevin de Bruyne to stabilize the midfield next season. And if de Bruyne doesn't play well, Manchester City's overall performance will suffer. This is why Guardiola needs Bernardo Silva so much next season.
Bernado Silva is one of the most players in Manchester City, but mind you that Pep Guardiola is famous for not standing in the way of players that make known their intentions to leave the club, so if Bernado Silva has decided to leave City, Pep wouldn't stop him. City have already signed a midfielder this summer in Kovacic, though he isn't a direct replacement for Bernado Silva, but he is going to bolster City's midfield. I don't think Bernado Silva is going to leave City, but if he does they'll find a replacement for him and City would still be very strong to compete for all trophies.