After a team rises in the rankings, the value of the players in that team increases significantly. These players naturally transfer to bigger teams. But after these transfers, the team goes into decline again. Finishing in 2nd place last season was a great success.
If the players go to other teams, the efforts of the whole season will be in vain. However, if the team management makes a few new transfers and ensures that the quality of the team remains high, we can watch a very successful team in Europe next season
It all depends on the team's strategy, if the team worked purposefully, achieved good results and in the end was able to profitably sell several players and thus earn good money, then perhaps this was their goal. At least I can easily name several teams that work exactly according to this algorithm and feel very good. But there are teams for which this is unacceptable (for example, Bayern), only victory is important for them, and it seems that Alonso wants to make Bayer such a team, although he had the opportunity to lead Bayern...
However it also depends on the financial situation of the team in my opinion, if the team needs money to make them better then maybe they won't pass up the opportunity to sell their players for a high price. On the other hand, if they have money and are more concerned with achievement, then they will try to keep him in the squad. The reason is that there are also teams that have stable finances but they are still busy doing business and put profit as their main goal, such as Borussia Dortmund.
Stuttgart this season has looked extraordinary and I didn't even expect that they would be at the top of the standings and finish in a better place than Bayern Munich and Borussia Dortmund. After the previous season, they even performed disappointingly in the Bundesliga. What worries me is where they will decline in the next season, because usually many teams show things like that. However, I hope that Stuttgart can maintain their good performance and be able to compete again at the top of the Bundelsiga standings next season.