The mining difficult adjusts. I don't know enough to say what it would take to drop so low that a single asic somehow was too overpowered to be economical, but generally there is automatic matching between the supply and demand of mining power.

As mentioned, 50% attacks are possible at some point (they may even be now). There is not really an incentive for that. I would be curious to see some analysis of how likely they become as the hash rate drops, and match that against the hash rates supported to profitably mine for different prices.