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When ASIC reach a cheaper point and everyone can get one then theres next step in BTC. But the Difficult will only increase, pretty similar to the jump CPU>GPU mining.
Right now GPU miner won't drop, if they do the difficult would go down and GPU miner who don't drop will have more BTC.
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When GPU miners drop out of the network en masse, their cumulative hash power will likely be a drop in the bucket in comparison to the widespread ASICs, so difficulty won't drop enough for much of a change in profitability for lingering GPU miners.
I do agree, however, that we will see a similar type of shift as you point out with CPU>GPU mining. With the only difference, and certainly a considerable difference, being that the ASICs aren't as easily re-purposed and sold (if at all) for recuperative costs. All profits are left up to the performance the ASIC had earned.