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.
When a soldier leaves a barrack, another one will be replaced with the ones that left, you can see that other clubs are already making moves for Chelsea players, Hervertz is one of the most sort players Arsenal are looking to bid one, they should come to terms by next week if things go well between Arsenal and Chelsea. If Chelsea is able to discard some of these players, it will do them a lot of favors to have big space to bring in new players that will help the club achieve its aim by next season but I think Pochetino has a big task ahead to forge the players into good tools, last season wasn't only the coach but the players also contributed to the club failures.
As for Arsenal being interested in Havertz, that's pretty good, it's just that this will get complicated when Chelsea start increasing their transfer fee. We know Chelsea don't want to lose this time because they have already spent so much on buying new players and the total range is 500 million euros for last winter.
Now Chelsea are going to sell some of their players and it's not surprising that Arsenal want to bring in Havertz, of course they have a good strategy to maximize the talent they have. I still don't know how much transfer fee Chelsea want and if they want Havertz with the equivalent of over 50 million euros I think it's still too expensive.