I'm not sure if I understand your question but Rashford has played at a high level and been at a level close to the best attackers in the league, so if you want to explain our relative lack of success you will have to find it somewhere else. The easy place to look is the overall strength of the team outside of Rashford. First the team lost the best manager in the world and arguably the best manager ever, secondly it's clear that United has lacked quality in multiple areas compared to for example our last CL finalist team, we haven't had the same depth in midfield and attack. None of the issues that the club has were caused by Rashford playing 300 games, if anything Rashford being able to play at a relatively high level from day one is a blessing, we basically benefitted from a Nani level player without having to purchase him or developing him into prime Nani, he magically appeared that way.
If you want to complain about something it will be about not having a prime Scholes, Carrick, Keane, Rooney, Tevez, RVP, Ferdinand, Vidic or Giggs. I won't even mention not having Ronaldo because that one is quite obvious and I won't add the rotation players who were almost all better than what we currently have.