For instance, I'd argue the point where Windows phone/mobile lost the "bottom" segment of the market (esp. in India and "emerging markets"), when the OS became too hard to run on cheap handsets of the lowest specs, to cheaper Android knockoffs was a key inflection point where the platform lost so much of its momentum.

Whether or not you feel that Microsoft buying Nokia was a cause of that or a symptom of that. It was somewhere between "both" in my mind, in that it was a symptom of software upgrades at first but snowballed because of the Nokia purchase. Certainly there were factors that the OS was maybe too quickly pushing bottom level specs. (As much maligned as the app lifecycles of Windows 8 apps were from a developer standpoint, they did help the platform hit some very low specs. Windows 10 over-correcting from developer complaints did not help mobile.) In addition to not solving the telecom carrier software update problem satisfactorily to OEM needs (as I mentioned), there's also the Android price match question of whether or not Microsoft dropped the OEM price to "free" for Windows on the mobile form factor too late to win new OEMs. (They probably were too late, but there still seemed opportunity at the time they dropped the price. Again, purchasing Nokia confuses the timing of all that and which effects were snowballing.)