Early on I expect FPGA to possibly end up being very profitable.  Some will sell their GPUs and re-equip with one or two FPGA at the same cost basis.  Some may quit entirely.  It'll be fine for a while.

The thing is people are greedy.  They are going to do the math and figure out how 10 more of them will vastly increase profits.  Which it will in the short term.  They will then take out a loan or whatever it takes thinking they have a gold mine on their hands.  And they will - for a while.

Once enough people do that, and once enough of the big players re-equip, then hello 10x difficulty increase.  It won't happen overnight.  Long term though - doesn't look real favorable to me.