Fab isn't the real issue. ARM Holdings doesn't have a fab. They license IP in HDL form to people that have fabs. And those people with fabs would like not to pay license fees. So why do they pay? Ecosystem.

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.