> it still is a much better starting point than coming up with your own bespoke ISA

5 years ago I would have agreed with this but now I'm not so sure. We live in an era where you can tell a robot "Here's some C code. Design a 64-bit ISA, write the Verilog to implement it in an FPGA, write a C compiler for it, and use it to compile the C code I showed you earlier."

And now your ISA and your compiler are part of your moat. I can just see the VCs salivating.