Until some day that one of those altcoin programmers discovers that his CPU has some interesting instructions like: FILD, FIST, FDIV, FSINCOS, etc.

Then the hours become months, and even after that the FPGA will have hard time beating even a cheap Atom CPU.

I tried to research exactly emulating 80x87 couple of years ago. There was exactly nothing available open source, with exception of an exact re-implementation of FDIV in Mathematica. The available closed-source code was not only expensive but had mandatory royalties and defense-grade NDA requirements.