"What's next? Windows 11 will be based on 7 UI?"
That is about the only thing I can think of which would get me to remain as a Windows user after W7 support dies. Along, of course, with the ability to permanently, completely, verifiably switch off all telemetry, spyware, nagware, updates bugger-ups &c. Doesn't seem very likely, somehow; but I'll be damned if go anywhere that unreliable pile of clumsy UI and spyware called W10.
As for browsers, although I'm a grizzled old-school coder among other things, I've never much bothered to interest myself in browser rendering engines. It's always seemed slightly pointless to have significant differences between browsers at such a low level. We have, or ought to have, a very clear and precise worldwide standard for HTML, agreed by all, and equally precise and unambiguous rules for how it is rendered so that the only remaining question is whether the code that does the job is high performance, high quality and above all secure. I guess I assumed that by 2018 convergent evolution would have produced a winner, almost certainly open-source, and that differences among browsers would have been based on stuff like footprint (heavy, feature-rich, ok for desktop; or light, slimmed down, great for mobile) and UI customisability (from very basic, not much you can change, to almost infinite choices right down to preferences of, say, automuting some sites*¹ and not others). My analogy might be old TV sets: from small cheap monochrome with three controls to a big colour console that even lets you adjust the saturation, the core of how they work is always identical (heck, they even made the colour 625-line PAL signal backwardly compatible with black&white!) withbthe same processes being used to extract the same basic image from a complex signal, and the real differences are built on top in terms of bells, whistles and expense.
Google is rotten to its core by this point, but if the core rendering engine is open-source I'm not sure we'd have much to worry about. Much as I like Mozilla for being not-Google, the truth for me at least is that Vivaldi offers by far the best browsing experience (Blink engine, and many critically useful tiny touches, like the ability to zoom a single page and not an entire site). (And of course, Firefox is unusably awful on Android, where Brave [Blink engine again] does a fantastic job.)
*¹ Yeah, I'm looking at you, CNN.
Biting the hand that feeds IT
