So let's say your team has what it takes to design and validate the HDL for a processor microarchitecture that implements some un-encoumered ISA with performance that is attractive and a sufficiently short errata list. Then published the HDL under some free-as-in-speech license. You still have to get gcc, llvm, and gdb to support you, at a minimum. And then you want the hardware debugger people to support you. They will support you if you get enough users, if you get enough support from tool makers you will get users. The trick is not running out of money before the ecosystem spins up and your competition releases a more attractive core.
(having done this ....) getting gcc/gdb/linux/etc working for your new architecture is something you will have done long before release your gates - both on your architectural simulator and you've probably booted linux at least once to a prompt on your gates (or at least HDL)
I've been on both sides, I found bugs in the original 29k bringing up unix (pre-linux) on the architectural simulator that were in the real chip, and as a logic designer helped design a new CPU and bring up linux on it
Anyone who seriously has a new CPU already has that ecosystem in place at the point they announce their CPU (and is probably busy trying to push their changes upstream to the gcc/llvm/glibc/uboot/linux/etc trees the day they announce to prove they are real)
But I agree the trick is to not run out of money ....