He's not very good as a playmaker or creator on the wing either, yet we play him there. Every player has their weaknesses. Rashford's are the ones you mentioned. But he still has 11g and 5 assists in 15 full games as CF. He clearly does the job.
He doesn't need to be great with his back to goal if our main focus is to get him to score goals himself, instead of to bring other players into play to score. In the same way that his lack of playmaking, decision making, ball retention are not a problem on the left, because his job is not to create but to be in a position to finish chances.
If you bring a striker to be the main focal point of the attack, Rashford's numbers might plummet again like they did with Ronaldo last year. He's not a provider, he's a scorer.