It is a pity that Frenkie de Jong ultimately stayed at Barcelona. He also seems to be a kind of 2nd choice there and will often start on the couch. At Chelsea or United, he would certainly have become a base player.
Although he is the second choice in Barcelona squad, he can play with quality players. He also seems suitable with Xavi tactics, so he feels comfortable staying in Barcelona. While if he moves to Man United or Chelsea, he will be the main player but may not be suitable with the tactics of the coach on these clubs. He also may feel uncomfortable with a bad situation the new clubs, especially Man United or Chelsea. These clubs are in a very bad situation this season.
With most chances he is going to be second choice for Barcelona because Xavi and management was serious about offloading him in last season but due to few unknown reasons this not happen, but he has done good job and having good play for his favorite club with current situation at Chelsea and Manchester United we can say this is good decision from him because both clubs are struggling badly and having bad time with performance and settling players due to new coaches system which is not working for many players, and they are unable to give their best.
But one thing is surely as Barcelona is doing hopefully they will settle with errors which are having win Frankie De Jong and try to offload him in coming transfer window as now Bayern Munich is also interested, and he can go there for better future.