SharePoint Online’s user interface has been rewritten in recent years using an open source stack (node.js, React etc) and certainly works far better using Chrome and not IE. This is true of SharePoint on-prem too, at least the recent versions. Even the tools we use to make web parts now are React, Yeoman, webpack, Yarn, Sass etc. Microsoft are heavily investing in these and most of the training materials they produce for developers are pushing us more and more in that direction.

In my understanding, Corporate is tied to IE for three reasons: it is easy to manage via central policies, it is reliable at rendering legacy (esp. intranet) apps, and Edge isn't even available for some Windows editions (Enterprise LTSB).

Especially the second point isn't new. So in the past, MS has built compatibility modes into IE, where a modern rendering engine is used by default (or at least when a certain X-UA-Compatible header/meta tag is present) and a fallback engine is used upon certain triggers (explicitly via compatibility lists and X-UA-Compatible, or via heuristics, e.g. for intranet sites).

I don't understand why they couldn't do a similar thing now: use EdgeHTML in IE by default, and fall back to Trident via certain triggers.

Yes, it would have required using two different rendering engines, while IE 10 and earlier can be emulated by IE 11 Trident. I don't know how IE is built, but heck, in the early Firefox days, I used to have an extension that could seamlessly switch to IE rendering within the Firefox window. So I would like to believe that a better solution than making IE the new Walking Dead among browsers should be realistic for MS.

Not a surprise, because it is created by Microsoft itself.

Yeah, even if officially deprecate it, this thing is going to live on for a few years still.