Ten Hag must win the dressing room because everything starts from there. Some of Manchester United's senior players look selfish in the game and actually this should be resolved rather than bringing in lots of new players. I agree that there are certainly some players who come and go, but improving the performance of existing players must be the focus at this time.
I agree with your point, Manchester United already have a lot of talented players and adding more players when they can’t manage and deal with the current ones will make the existing feel unappreciated of that fact and assume the team don’t really value them for bringing in young players like them. The management might also want to make the team more competitive by bringing in rivals to those players that don’t perform well or are full of themselves. Whatever they do or intend to do, they should work on improving the overall performance of the team because they are lacking in all positions now which they need to address before it gets worse for them in the league.