Well I think GW did a smart move here. With free rules and cards for existing models, there is no reason for current WFB players not to give this game a try. That being said, it is not going to occupy that same space as WFB. However, there are options out there. A) keep playing 8th, B) there are people out there creating something called the 9thAge, basically a tweak of 8th, C) yes, oh yes, there is Kings of War that has a new edition dropping in August.
Now, as to AoS, what really concerns me about this "model count" balance system, is that how do you handle horde armies vs elite armies? If you bring 50 chaos warriors and I bring 50 zombies, I can't imagine that is going to be anywhere close to a fair fight.
And clearly, we all know that GW's new factions and units are going to outclass most of the old stuff right from the start. That is just how they roll. I don't see AoS having a long life with people using existing models. The old armies just aren't designed that way. I think AoS will develop its own following using GW's new releases to sustain it.
If you end up not likeing AoS, or crave the massed battles game, then start looking to one of the above options. I think the 9thAge could turn out to be a really cool Living Rulebook of 8th edition. I enjoy KoW, but it doesn't have the same depth as WFB did.