Ah interesting! I work on (very expensive) SDRs and we make pretty heavy use of Xilinx Zynq Ultrascale SoCs. They combine Cortex-A, Cortex-R, and FPGA fabric all in one package, with some fancy interconnects. So you can handle the hard realtime stuff on an RTOS or in the FPGA, then send the data over to the application processor with a hard float ALU to crunch some numbers (or build some kind of dsp IP into the FPGA, idk much about that side of it).

I've also seen some cool stuff with the BeagleBone products, which have a few TI custom architecture DSPs and "realtime units" which you can communicate with via Linux.

But yeah, I can certainly see how just doing it all on a super fast MCU could be easier and cheaper without the backing of commercial enterprises.

I've always thought it would be cool to design a "poor man's zynq" hat for a SBC. Stick a RP3050 and a Lattice FPGA on there and set up some SPI / UART connections.