There is a thin line between being patient about a player's potential and holding a young player back unnecessarily so whilst what you say has some merit we always have to consider that this young man took his chance where countless others fail so we need to reward that. Sending him down to the U21s or on loan to some Championship team is not a reward, keeping him in and around the first team, obviously with reduced responsibilities, is a reward of sorts because it gives him a chance to stake a claim on more prominent role. It doesn't mean that he is above playing games for the U21s but that he has immense potential which can be utilized now.
We have Rooney and Martial as our two senior strikers and they have both 'skipped the steps' you are putting so much import on, that's not my point though, but that having so few strikers present opportunities for Rashford and Wilson to stake a claim for that third striker role whilst, obviously getting games at U21 level.