If monsters were unable to cross channels, once again, then Madina, D'Hara, and Astrum's survival would not longer be at stakes, just their expansion into Candiels, Paisly, and Eidulb.
There'd just be the issue of Avernus's land crossing. But unless the code intentionally tells the monsters "go east, guys!" (which it might, but shouldn't be hard to change), wouldn't be all that much of an issue. First off, there's Westgard in the way that tackles on 10k-30k hordes almost daily, seriously denting anything heading North-East, and secondly... would monsters reaching Avernus really be that much of an issue? Avernus lacks the history of Astrum, D'Hara, and Madina. As does Arnor. And that quasi-island is a perfect monster killing ground: one entry on both sides containing a large landmass to diffuse any force that breaks through with a ton of fortifiable locations. I don't expect much monsters to be able to make it past the meatgrinders of Westgard and Nifel's quasi-island combined, as long as the code doesn't intentionally seek to make this happen.
