Whoever has worked with FPGA knows that they are completely different to program than CPU/GPU.
They are not competing at all. You can't take some computer developers and have them work with FPGA. That'd be like taking a dude who knows XML and put him on optimizing C++ low level algorithms.
For starters, a FPGA doesn't run programs, it describes hardware components.
Hardware engineers can design FPGA-based hardware optimized for ML.
A second set of engineers then uses these boards/FPGA's just as they would GPU's.
They write code in whatever language to use them as ML co-processors.
This second group doesn't have to be composed of hardware engineers.
Today someone using a GPU doesn't have to be a hardware engineer who knows how to design a GPU. Same thing.
Bitcoin is a good example. I'm reasonably sure that FPGAs eclipsed GPU there because some "software person" called the right person in. And it certainly was a competition. Similar for the eventual move to ASIC.
If there's enough incremental benefit, it will happen.
[1]: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.50....
I also believe that today's FPGAs are more robust against these kinds of bugs. Because, even with some HDL code, these defects or cross-talk incidents could result in difficult to debug errors.
And if it is just about bang-for-buck and scale, FPGAs seem to be quite competitive.