It will happen at some point. One possible scenario is the cost of mining goes so high in terms of new hardware required to keep up, and the power cost, so much that ASICMINER goes bust, switches off, and hundreds of TH/s suddenly disappears from the network.
Not singling them out, this is a serious possibility with any BTC mining consortium that needs to make a profit to survive.
90% is unlikely no pool is holding a high hasrate like that, no farm either, 10% might be but it's hardly a factor in this, in fact when knc miner went bakrupt nothing really changed, and they were big enough at that time
This has happened with a number of altcoins recently. Basically the only solution was for them to patch the client and hard fork.
we can't even hard fork for 2mb , you think bitcoin will hard fork for this? the consensus will never reach an agrrement