You are right, Osimhen would definitely be more useful if he joined Manchester United, because in Napoli he can be said to be no longer considered. Not because he played badly, but it was all caused by personal problems that Osimhen had with Napoli. So that's why Napoli currently does not make Osimhen a main player anymore, even this season Osimhen was loaned to Galatasaray. That's why rather than Osimhen's fate being unclear at this time, I think joining Manchester United is the right solution.
Rasmus Højlund is not a good striker while a big club like Manchester United needs to have a top-notch striker who is goal productive and stable in performances. Osimhen can help them to resolve issues at this important position but Rasmus Højlund can be kept and used as an alternative striker for Osimhen. During a very long season, Manchester United need to have more than one striker and Højlund is an acceptable substitution for Osimhen.
Rasmus Hojlund is on the selling list, not only because he is considered a failure at Man United, but Man United is indeed in need of additional funds for their project next season. So, if there is a team interested in recruiting Hojlund, it is likely that the offer will be accepted by Man United. Osimhen is a good striker, but he is not cheap, so Man United needs more money. Gyokeres has also been linked with Man United, and the same with Osimhen, Gyokeres also commands a high price tag.