It is adjusted, but by an algorithm shared among all clients, rather then arbitrarily by a person. Every miner and relay client calculates the difficulty themselves, and refuses to accept any block that was not solved with the correct difficulty. The difficulty changes after a fixed number of blocks rather than a fixed amount of time.
A sudden influx of ASIC miners, carefully timed, could disrupt things a bit for a short time, but it isn't very practical, and I don't think it would disrupt the overall market much. If they successfully sped up the overall block rate, they'd just make the difficulty adjustment happen even sooner.
The only case that might be a potential concern is if >90% of the miners all shut down at once. Then, it could take quite a while to solve enough blocks to get to the next difficulty adjustment. I can't forsee a situation that would cause that many miners to all shut down within a few days of each other, though.
OK thanks for explaining it better. My initial impression was that everyone was just brute-force attacking the same mathematical problem and there was only 21 million solutions. Apparently it's way more complicated than that.
A sudden influx of ASIC miners, carefully timed, could disrupt things a bit for a short time, but it isn't very practical, and I don't think it would disrupt the overall market much. If they successfully sped up the overall block rate, they'd just make the difficulty adjustment happen even sooner.
The only case that might be a potential concern is if >90% of the miners all shut down at once. Then, it could take quite a while to solve enough blocks to get to the next difficulty adjustment. I can't forsee a situation that would cause that many miners to all shut down within a few days of each other, though.