That's due to a lack of demand. The ultimate problem for high security. Gaisler's stuff is already implemented. The RISC-V Rocket core is 1.4Ghz. Basic checks like in SAFE's atomic groups have single-digit, percentage effects on performance. The mods can be tweaked to give desire performance. Worst I've seen was 70-80% loss on a prototype designed for research rather than performance on most memory-intensive stuff. Not 100x or anything.
"Meanwhile, the fast machines available to us all have opaque, cryptographically locked, and seemingly often vulnerable supervisory code running on them."
Definitely improvements to be had in things with open firmware and such. POWER and SPARC have had that for quite a while via Open Firmware standard. SPARC and MIPS have open implementations with SPARC not requiring an ISA license. Not sure about availability of server-grade POWER implementations or licensing costs. Definitely need more to work with in that area.
"To make an impact OpenPower solutions need merely be better in some respect (avoiding the secret supervisory code) and not too worse in others."
I hope that's true. What killed it before was legacy effect: key apps or capabilities that worked on x86 didn't on alternatives. Even Apple abandoned PPC. Only one doing it on desktops are the Amiga systems.
For common use-cases, we need to port Flash, JS JIT's, ASM-optimized codecs... all sorts of things to POWER to achieve parity. I've always liked POWER better than x86. It's just that users use apps not ISA's. That's why x86 has holding power.
So, it's literally gotta be users who don't need games, fast browsers, CAD, etc. It has to be people using native apps written in portable code leveraging components written similarly. Even better if it's multi-threaded or benefits from POWER's accelerated instructions. How many use-cases does that represent for desktop users?