I think part of the goal for Turbo9 is to be used in the real world. By using an established ISA and maintaining binary compatibility, their CPU can be targeted

I think part of the goal for Turbo9 is to be used in the real world. By using an established ISA and maintaining binary compatibility, their CPU can be targeted by any tooling built for it over the past 40 years. Think of it: all existing documentation, code, compilers, assemblers, know-how, even existing compiled binaries! As soon as you create your own architecture, you're essentially a toy. Nobody will want to rely on your half finished C compiler for it, or your unmaintained patchset for LLVM, or be willing to learn verilog just to fill gaps in documentation. Even something as "simple" as changing registers on RISC-V will break most tooling and make most existing documentation impossible to rely on. Resulting in essentially a new architecture with a confusing name. Being a toy isn't a bad thing, it just wasn't their goal here.