is everyone designing their own silicon getting so much additional them-specific utility out of it that it's actually worth it?
Possibly. Realistically this is replacing the expensive category of FPGA (Zynqs or similar with strong hardware CPU cores), this means they get all the peripherals they desire in hardware, and they can pick the core variant in order to optimise for their workloads (all the different vector extensions for example). There's an interesting market for that kind of thing, either full FPGA to ASIC replacement, or drop in replacement FPGAs of lower cost (The Rigol MHO98 replaced the Xilinx FPGA of the previous generation with a substitute from Fudan). If you're shipping a lot of hardware, that sort of thing becomes worthwhile.