Excellent concise summary, glad you and
ForOldHack beat me to it. I would have confined my reply to the ring issue having forgotten much of the controversy with other OSes—Netware for instance (even though I'd used it many times).
You're right about segmentation being hard work, I can recall cursing Intel at the time and hoping it would bring out a more eloquent processor that was easier to use such as Moto's 68000. However, there wasn't any thought of moving over to Moto because of existing investment in Intel's development systems-Multibus, and in the 8085 and 8051, they were still current and we were still using them.
"It's a huge waste of human effort:"
Absolutely, if I could have that time over I reckon I'd opt for a different profession. Everyone from manufacturers to end users wasted a huge amount of time and energy that resulted from stupid mistakes to outright shenanigans. We saw it all happen from a developer/end user's perspective and we weren't immune either (also it ended up costing everyone unnecessary money).
For me, the most bitter arguments were over the change from IBM to Microsoft which also included the shift from OS/2 to Windows. Technical arguments became irrelevant after senior management read press reports that Microsoft was the new 'in fashion' company when it came to computing. I recall saying to management "why waste all the expense on an engineering outfit when you can simply find solutions in the finance section of the newspaper". That didn't win me any Brownie points.
Why so many in computing and tech became so besotted with Gates and Microsoft and why more level heads never prevailed at the time has always troubled me. For instance, when Gates was quoted as saying "640K ought to be enough for anyone" I recall saying to colleagues who agreed with me "either Gates is daft or he's been forced to read from a PR release". Whilst there's now doubt that he ever said those words there was never any outright denial at the time. Similarly, the technical press never went out of its way to say how stupid the notion was. Being obviously silly or at best ill considered then why didn't the press say so? There's shades of 'the emperor's new clothes' here, methinks—and that's troubling.
I'm of the opinion that long after all protagonists of that era are dead and buried that researchers will tally up the actual cost of that wasted human effort and we'll find that whilst Microsoft wasn't completely responsible it nevertheless played an overly dominant role. The same applies with Windows and MS's deviations from the original CUA (which are still ongoing). Just think of the millions of hours lost worldwide by users having to relearn and futz with the UI whenever MS changes it. If dollar figures were put on this wasted human effort then they'd have to be staggering.