He would have had the majority of land, but not the majority of population or the majority of wealth. There's also the unfortunate fact that the North is going to be eaten by wights when the wall comes down.
That's not really a viable argument from an in-universe perspective. Nobody involved in the War of the Five Kings (prior to Stannis) either weren't aware of or didn't care about what was going on beyond the Wall. If Robb had known that Mance Rayder was massing an army and the White Walkers were returning in force, then yeah, I'm sure he would have put aside his desire for revenge and focused his efforts on preparing the North for that, instead.