It's not just about selfish or unselfish mate, it's about skillset there too. Rashford is not a provider, because it's not in his skillset not because he's selfish (although he is a bit of that too). Rashford isn't Grealish and Grealish isn't Rashford.
For example I'm not saying Osimhen is selfish, but he's clearly the kind of striker who shoots on sight and is more adept as the finisher of the team, as opposed to someone who brings others into play like Kane. A striker who likes to play on the shoulder (like Rashford) instead of drop deep, or a striker who's more likely to take a shot than eye a pass, is going to lead to diminished output from Rashford. And it has nothing to do with instagram unfollows and pettiness.
And Rashford scoring less might be all good so long as we're better as a team for it, it's a team sport, but it's not certain. There's no point in transferring Rashford's goals to a #9 if only for Rashford to regress to 5 goals a season again. We'd be back where we were.