Windows mobile wasn't going to change the world. That said, I'd love to see a real alternative to iOS and Android if for no reason than my phone is a very powerful, very real computer and I can't use it as such.
I can think of one, but in the opposite way: how Google removed the Exchange integration service from Android.
And then of course, you have both Google flirting with Android apps on Chromebooks, and the new M1 Macs being able to run iOS apps, when Universal Windows Apps were a thing over five years ago.
They've all had this a little while, but Microsoft was ahead of the game on being able to allow/deny individual permissions to given apps before either Apple or Google was as well.
I mean, Microsoft didn't have the market capture, but their platform was technologically in 2015 where their competitors are in 2020.
The long term vision is exactly what you said, convinance & security of a smartphone with, for those who wish, the openness and power of a computer