Good question. I'm not including GPU miners leaving, since of the ~ 24Thps that were present prior to ASICs I don't know how much was attributable to GPUs and how much to FPGAs. So I don't know how much the hashrate would drop once GPUs are unprofitable. Another thing is that I've very carefully avoided guessing what the exchange rate will be, which is needed to determine GPU profitability.

All I can say is that some point we'll lose GPUs and at another we'll lose FPGAs. Many GPUs will probably leave early to mine LTC, FPGAs will mine on until the bitter end. I'm assuming that by the time this happens, 24Thps will be a very small fraction of the network, and probably won't have much of an impact on the forecast accuracy given all the other sources of error (shipping times being the largest source of error).