There are too many players there so Chelsea need to make reductions and Pochettino as the new manager there where not as someone involved in buying these players will choose which one he thinks will be suitable as a starting XI and also a reserve so that his team is competitive. Mason Mount has long been rumored to want to leave Chelsea due to dissatisfaction with the wages being earned, with other players still around Chelsea should be able to cover well the place Mason is likely to leave.
Chelsea did spend too much money last season to bring in all the talented young players and they shouldn't have to be hasty in making these purchases if in the end Chelsea change coaches for one season. Now Pochettino's arrival will indeed give a new color to Chelsea and of course, as you said, every coach will only need players who are felt to meet the standard strategy that he will provide.
Chelsea's mistakes are indeed very difficult to accept and therefore now the players will start to be sold in the summer. Regarding Mason Mount, it seems that he is not comfortable at Chelsea and the salary does not seem to make him want to stay at Chelsea. There will be many replacements for Mount later and Chelsea don't seem too bothered by that.
Chelsea's problem may lie not in the players unfortunately, but in the club president, who has a very strange way of doing things, starting with the choice of coach. Buying players for 400 million euros for an average coach is a very risky idea, why in the beginning they did not find a top coach, who would also pick the squad is also a mystery.
In general, Poccetino will have a lot of work, but he knows how to work, he has character and he will manipulate himself, so maybe he will get something good this season, at least to return to the top five teams in the league, it will already be a success.