There is simply no point at which the difficulty will be low enough and electricity expensive enough to make FPGAs more profitable than GPUs!
Fucking christ you people need to buy more nootropic drugs and fewer bitcoins and associated paraphernalia...
Not getting through, let's try a simpler example.
If the price per BTC is $50 but cost to produce is $3 there is no real reason to optimize for cost of production. You will want to optimize for volume. If you could produce 2x as many coins at the cost to produce of $6/ea, you would do that. The actual difficulty is immaterial, what matters is the profit margin per unit.
If the cost to produce is $3 and price is $8 (as we see now) there is still little reason to focus on unit cost as the main optimization parameter.
If the cost to produce is $3 and the price is $1... Well, I've already gone into it, you're just not following along.