You are immune to logic sadly, but I'll answer two things, which can be extrapolated to all you've said.

Sometimes yes. Usually no, for most people. If you make a word document in an older version of office, it’ll still work fine. If you use LibreOffice with the oldest-looking UI, it’ll still work. 99% of people don’t use the extremely niche features that have been added in recent years.

No, it all won't work fine at your work where you send documents and spreadsheets and stuff with complex functionality to your colleagues and clients. And they send their documents to you. And versions edited in your old version or LO break.

That aside, WordPerfect 8 doesn't support MSW document formats, IIRC, and MSW doesn't support WP8 document formats.

Exactly. And that research has lead to where we are now.

This is factually incorrect and I have already said it's incorrect. That research has mostly been exhausted, and the conclusions one can make from it are more or less the same as in 40s, 60s and 80s. And 90s' interfaces were more usable because by habit people tried to follow industrial ergonomics, even though computer displays allow one to cheaply shoot their user in the foot, in the way some device's panel with switches, buttons and knobs doesn't.

They almost never were. Seriously. Go back and try some 90s software. Most of it was a cluttered mess, ugly, really weirdly laid out, and had zero considering for anybody with disabilities.

Some of it. But IBM and Apple had human interface guidelines based on actual research about ergonomics, which hasn't become obsolete despite what you say, because humans did not change as a race in 30 years. UI\UX following those is still good.

Judging by the first quote, you simply haven't done work requiring heavy usage of productivity software yet.

Also you are arguing like a schoolboy. Exactly in the way schoolboys consider to not look like it. I could give advice, but that usually only results in resistance.