Not sure if already discussed (too lazy to read through the whole thread right now, sorry).
How about GPU miners that might move away when profit is close to 0 for them?
edit:
Nice work, an interesting read!
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).
Litecoin has been consistently ~1.3-1.5x more profitable than BTC for GPUs for months now. I expect most GPUs have already moved.
(Indeed, a large % of LTC GPU miners were even switching between alt coins because they were more profitable still, but nowhere near as stable as LTC)