Manchester United currently on their top performance, those players have nothing to lose to join Manchester United because they will get a lot popularity and their performance are good in their current teams. Dortmund, Tottenham and Napoli are just mediocre teams, just use that's team as the step stone to join the bigger teams. I believe the players will get better salary too. However Frenkie De Jong have been joined one of the best team in La Liga, so he can choose to stay in Barcelona.
Up to the next level when joining Manchester United than keep staying with Tottenham Hotspur for Harry Kane and Victor Osimhen keep playing with Napoli until his contract expiry in 2025. Indeed this season Napoli have promising performance and potential with Serie A tittle but joining Manchester United become great opportunity and he will be more popular than staying with Napoli.
For Harry Kane I think become second chance for him after last two season rejected with Manchester United offering and keep stay for Tottenham Hotspur without any trophy yet during in his career, seems have bigger opportunity get tittle with Manchester United after having top performance and EFP Cup tittle is the proof how Erik Tan Hag can bring United more success on the next season.