I see the appeal here, but I'm surprised a good DSP or mid-volume, old node ASIC isn't the more common solution here. Do these need such specialized processing that FPGAs make economic sense?
Scopes and other digitizers can operate at GHz rates and something like an FPGA can interface those rates (JESD) with other peripherals like memory. (for example)
Scopes and digitizers are actually extremely demanding in terms of throughput, plus they do a lot of specialized processing on the samples. Even mid-range models are using top-grade FPGAs and custom ASICs. Keysight's mid-range scope is capable of 5 gigasamples per second at 8 bit sample depth on 4 channels, that's 20 GB/s of data that needs to be processed, [soft] real time.
Regarding cost, these are expensive, complicated instruments. A bottom of the barrel oscilloscope costs $300 and professional grade units are more like $2-3000. The top grade ones can cost half a million dollars ( https://www.keysight.com/en/pcx-x205212/infiniium-z-series-o... ).