Licensing aside it is still pretty bold for a fabless IP-only company like ARM to release this level of detailed specification; unlike e.g. Intel or AMD who could still have an advantage even if others made competing implementations, because of their fab technology.

I'd bet there already are clones/semi-clones of ARMs hidden in various obscure far-East ICs which have no user-accessible firmware... we just don't hear about them because the ones we do, would've been sued out of existence. I've seen presumably-unlicensed MIPS cores a lot in those.