I agree with everything you say and I acknowledge your background on the matter. All I was saying was that 'unusable' was a generality. It may apply to most people, as you said, but not to everyone. It has to be qualified.
I put out a newspaper, including classifieds, legals, layout, ads, etc, from 2004 to 2013 using PowerPC Macs. The only reason we switched to an MP in 2013 was because the LB on the work G5 died. I replaced it and upgraded my coworker from a G4 to that G5. She was still able to do everything I was. We were both using CS4.
I'm a graphic designer, so using my Quad G5 to lay things out and then export a PDF or whatever is required for the job is still doable. And I can tell you that all a print shop cares about is if the PDF you send them is right. They don't give a crap about what platform and what program made it.
CC 2020 also uses CS4 as its baseline for downsaving, making PowerPC still relevant.
Maybe design is a narrow use case for a lot of people, but I know there are still plenty of designers out there using these old systems.