OTOH, they'll be playable right out of the gate. You'll be able to put your armies on the table and play them against your friends. Yes, they probably will be outdated in a year or two. That's a real concern, but not one that WHFB players haven't faced even when that game was still alive. A lot of armies went a few years without updates. So i don't want to dismiss your concern, because it is a real concern! But I also think that this isn't so bad and is infinitely better that what we have now.
I also think there is a real possibility that if the game is successful they will expand into the armies they don't plan to support on release. they said as much with Chaos Dwarves, that they COULD happen after a few years. "Could" certainly doesn't mean will. But I think its pretty likely that the first few years will be spent just getting the initial 9 armies up to speed. Which is a pretty huge undertaking, even making use of old models.
I'm really not trying to be too optimistic. In fat, up till now I've kept my expectations extremely conservative. This is much more than I expected.