They fucked it up with uninspired design (a start menu and task bar on a mobile?!)
You mean how they designed the first button free mobile device that everyone copied after that? It's entire design was how to make a button free mobile device.
Phone makers didn't want to drop buttons though so you still had phones with buttons, which I think is what really killed Windows phones. That, and not being able to install devices beyond their store in their last Phone OS (it was possible before), the final nail. Trying to be more like iPhone there and less like Android hurt it I think. It's also after this when you start to see companies moving towards less buttons to no button. LG Prada, iPhone, Android, all moving on this design idea.
MS was building some great designs and ideas but with only doing the software, the hardware it was designed for wasn't being made. It's why I think Microsoft made the Surface division, so they could make sure to show "this is the newest X we have designed and how we envisioned it".
If you're talking about WinCE/Pocket, etc, it was an extremely bad UI paradigm for a phone and a button free design in this case made it worse, not better and no one copied that especially not after the iPhone was announced and shown.
The last iteration of Windows Phone (eg: Metro) was actually quite good, but wouldn't have existed without iPhone/Android before it. It being more like iPhone wasn't what hurt it, what hurt it was that they never got the dev support needed. My wife had a Windows phone for around a year, and the thing that ultimately moved her to iPhone wasn't that she didn't like the phone, it was that she was constantly left out of things because it was probably more rare for an app to hit Windows Phone than Linux.
Microsoft did have the right idea with getting to mobile/tablets before most, but MS has never really had good taste when it comes to software UI.
Part of their issue is their desktop and x86 legacy apps ecosystem was no use on ARM touch devices.
But more competition than 2 would have been nice. We need stuff to move back to mobile web apps instead of apps. Then it's platform independence and the sandbox is interchangable.
Fuck you, I loved the design of windows phone. Bring able to size the tiles different and have them show content on the home screen was awesome. And the hardware was cool too. I still look at the photos I took on my windows phone and compared to my galaxy s22 ultra they still look just as good if not better in some cases.
Honestly the wort thing about win phone was salty developers who not only refused to port apps over no matter how easy MS made it, but also went well out of their way to shut down any community apps made using their API, like the Snapchat dead did.
There was also Windows CE, which was a real shitshow. I had a Vadem Clio, which I still wish I had because I was a beautiful piece of hardware... but it was so hampered by having Windows CE installed on it.