At that point the economies make no sense.  At 3.3M difficulty, with your 250M FPGA you will be producing 0.08 BTC per day, so at $10/BTC you would pay off your FPGA in 625 days, in order to pay off your FPGA in the same 30 days as you can pay off your GPU currently BTC would have to be worth $208/BTC.  Because entrance to FPGAs is MUCH higher than GPUs that will push their usage back.  And you forgot to account for people with free power which may very well eliminate FPGAs usage all together.  FPGAs will probably never be popular for mining due to slowing difficulty and ever lower ROI.