I just don't know enough about POWER9 yet to make any kind of conclusion about it's suitability for other than Linux.

If Linux, the OS that is, runs on a single core and distributes tasks to other cores, which they refer to as 'slices', then perhaps a single threaded OS would at least boot on such a machine. (?)

I'm certainly not advocating that any OS team drop everything and go to POWER9.
However, that should not mean they should ignore POWER9 as an option. The A1222 should be developed and promoted for both MOS and AOS as it fills a gap in the hardware lineup that's been empty for too long.

If Apple had not killed the PA6T perhaps MOS would be on that. Just don't know.
And now Apple is planning another jump away from x86, and you can be sure if they go ARM it will be a custom top secret design that only runs their OS, so porting anything to that would be next to impossible.

POWERx. Is it an option? Certainly not up to me. Would it be cool? Hell yes.