I think Windows Mobile gave up just prior to the best chance of breaking the duopoly: PWAs. Most app developers will never want to support a half a dozen phone OSes. The solution will inevitably, eventually, be the web.
I'm not sure how much effort went into supporting PWAs for Windows Phone, I used to develop for that ecosystem during 8.x and the browser was a mess; cordova apps for WP required specific hacks to basic elements such as header, footer etc.
Besides, they where making good progress with supporting android apps in WP with 'Project Astoria' and iOS apps with 'Project islandwood'; both of those were pulled off abruptly. There was even a rumour in the WP community at that point that Project Astoria became too good that MS brass worried that native WP development may go extinct.
Btw, recently german WP enthusiasts managed to get Project Astoria working on several WP phones[1].
This is turning to "Linux for desktop" really fast. Or really slow depends how you look at it.
Even the web does not look like the web anymore. I am not even sure what do you mean by "the solution will inevitably be the web". The solution for supporting half a dozen phone OSes will be fewer phone OSes. If you think that nobody will want to develop half a dozen versions of the app natively, why do you think they will want to debug PWAs on half a dozen OSes?
