The only thing an FPGA wins on is NRE (non-recurring engineering).

The real problem is that the giga-sample DAC's/ADC's aren't willing to speak one of the actual high-speed interfaces or put a DSP directly on the ADC/DAC. So, everybody needs to use an FPGA to shoehorn the data into a useful form.

If somebody put an actual DSP on their ADC/DAC, FPGA's would evaporate for this application like they have evaporated for so many others.

Any FPGA application with volume eventually gets subsumed by special purpose hardware on a microcontroller. For example, people used to use FPGA's for PWM, motor control, etc. Now those blocks are standard on microcontrollers.