We have seen it with other alt-chains. There is a small amount of hype, a token amount of hashing, a lot of pump and dump and the chain dies off.
AH. That's a good argument!
I was considering a world with a large number of alt-chains (1000 and more) and a large number of miners (some 100M) and I was studying the theoretical stability. Of course these are unrealistic assumptions. With some 10 alt chains and some 12 guys working on them, social inertia is way large enough to stabilize the system. We'll see down the road if this changes as soon as bounty drops to 1 BTC or lower. Then, probably, new incentives could come up, if Bitcoin does not pick up as fast as most of us hope.