It is instructive to note where FPGAs are used (and yes, they are used). For example, lower-priced oscilloscopes. Too small volume to merit an ASIC, too high performance to use a CPU, and too fast to interface directly to the central DSP.

FPGAs really only make sense when you need to move a metric assload of data around in a hurry while simultaneously running "embarassingly parallel" logic... and even then, they are rarely the most economical approach in the long run. As a rule they are replaced by ASICs in specialized high-volume applications and CPUs in less specialized ones.

There's nothing better for prototyping and proof-of-concept work, though. And they'll always have a place in low-volume applications that aren't cost-sensitive.

They are also replacing TTL circuits in small volume command and control applications. Circuits with dozens or hundreds of 7400 series ICs wired together in Byzantine ways.