To be fair, Mbappé scored 29 goals or whatever it was as an 18 year old. His projections therefore present a different question. The question with Rashford is 'will he be able to develop into a 29 goal striker', whereas with Mbappé, it is 'will he continue to be a 29 goal striker'. In the case of Mbappé, there is already tangible evidence that he can do it, with Rashford, we are hoping for the unseen.
I think, provided you are satisfied with the hunger and attitude of Mbappé, which by all accounts, is first-class, at least on the pitch - then you would expect him to continue, and perhaps even better, what he's already done.
Mbappé is the best young striker I think I've seen since R9 and Owen. I strongly doubt he'd fail. With Rashford, the conversation is still a lot of 'when he adds x or y to his game, he'll be this and that'. With Mbappé, it's already there to a very top level, and you would imagine it can get better.