Yeah, but there will be probably thousands of levels for free and people will play them, so it could affect someone buying another Mario 2D platformer as he just played enough of it.
I would buy the new Mario game, as I don't think I will be playing these Mario Maker levels, as I don't like just going through it without any reward at the end , but some people maybe don't care and will at least postpone the buying of new Mario game.
I know that the new Mario will come on a new platform so the Mario Maker shouldn't really affect it, but because it seems like it will be not really powerful, I am not sure how much better will the game look compared to latest version of it.
But again, it's a Mario, people will probably buy it .