It's a possibility, sure, but at the same time, people still contribute PRs to Unreal as well, and there are lots of people that just toss stuff up for free to the Unity store, even though it's possible to sell it on the store instead.
Some people just like contributing. It's possible that major, shifting features could become less common in the face of the opportunity to sell it, but there's definitely people that don't especially care about that, and practically no one is gunna try and sell a bugfix or the like, so at minimum, fixes and minor improvements would likely be just as common as they are now.
That said, as a whole, I think it's probably for the better to at least have that avenue. For people that aren't as comfortable with core development, such as Duion, being able to pay a bit of money to pick up a game code starter kit would make perfect sense, and that isn't the sort of thing that's likely to be rolled into the core repo.
Same could be said for a programmer that could make the game code, but needs some art and the like.
The only potential hangup a store could see is pertaining to major features implemented in the engine code side of things, and as I said above, I'm not sure that it'll be as drastic a shift as some may fear it is.
Especially if we have alternative payment methods like being able to hook to funding campaigns like what Rich has going or tip jars.
But there will always be some people that just like contributing stuff for free, either because they like the challenge, or want to make it so others don't have to deal with some crap they had to put up with, etc.
and @ Azaezel
Yeah, the sub-forums thing Az kicked this off with is probably what we can go with as a compromise for the near-term until we can hash out the store enough that it works, since as I mention it's not there yet.
And even once the store DOES go up, those subforums can still be used to highlight products put up on the store, or freebie contributions/tip jar/buyout projects.