As far as I know, the Abyss isn't that big of a deal in 4E. It's pretty much just where the demons live (along with some deities and other assorted horrific creatures). So without the Abyss, those things just live someplace else. You seem to have figured it all out pretty well already. The demons don't really care about the Abyss, and they don't mind living closer to where the meals are, so there's no problem there.

Although, if you have packs of demons roaming the countryside, that's gonna push the civilized races into greater and greater isolation, so really play up the "points of light"/dark ages aspect of the setting. Enough demons putting down roots on the Prime really will be like Hell on Earth (or the Abyss on Earth ), and that's going to have some effect on the culture, politics, and economy of the area. People will be even more distrustful of strangers, justice will be even harsher than usual, etc. This really has a lot of potential to become a world of extreme opposites if you want it to.