Osimhen had only one good season, after that his results started to decline, and there is no guarantee that he could score goals at Chelsea where there is a lot of competition. Chelsea considered Osimhen only to get rid of Lukaku, but since Napoli bought him, Chelsea lost interest in Osimhen.
Osimhen actually got two good seasons, one before the season with Scudetto and one with Scudetto. The first season was broken with his injury but it's not true to say he played bad in that season. The season with Scudetto was simply magic, nobody can deny it.
Chelsea did not want to satisfy the super expensive transfer cost required by Napoli and if possible, they wanted to use Lukaku as part of the deal which was never done by two clubs.
It is not right to compare Chelsea and Manchester United now, because Maresca has just joined the team and they are at the formation stage and have a chance to become better, and ten Hag has been in the team for several seasons and in my opinion their results are only getting worse, so it is difficult to talk about any progress or improvements here. If you want to compare their achievements over the past 5 years, then Chelsea won the Champions League in 2021, by my opinion this achievement will be higher than all trophies...
Last season was good enough for Chelsea and Pochettino but the Chelsea's owners and the coach failed to find a common plan for future, so they separated each other and Maresca came, with a lot of uncertainty about his management ability and his plans with Chelsea.
It's not too fair to compare Ten Hag and Maresca, Manchester United and Chelsea, but if I make a comparison, I saw things are worse in Manchester United. Because Ten Hag was there for more than two years and we can clearly realize his failure. Will Maresca succeed with Chelsea, I don't know but he has a lot of time for doing his job and proving that he can be a good coach for Chelsea.