Once everyone sells their GPUs and moves to FPGA your electricity savings are wiped out as is any advantage in hashing power you had. Difficulty skyrockets once everyone re-equips. It'll be good early on in the transition phase assuming this actually happens on a large scale. Past that not so much. Balance returns and you've come full circle.
That's not true.
If you replace a network hash for hash you end up with the same hashrate and 1/10 the ENTIRE NETWORK POWER CONSUMPTION.
If you replace a network board for board you end up with the same power consumption and 10x the hashrate which provides increased ASIC resistance. This makes any ASIC above 45-32nm not profitable and not worth the effort in making. No more private / secret high level 90, 110, 130, etc nm asics!