I really liked the Windows Phone UI too - but christ it was buggy! In true old-school Windows fashion, it frequently crashed and required a reboot.
When Windows Phone died and I switched to Android, the UI didn't feel as intuitive as the Windows one did. And of course it doesn't help that Google change the UI, in particular the settings, all.the.time.
I wasn't a fan of the UI, but I can still appreciate the speed and fluidity of the phones I saw my friends using. We certainly need more than just iOS and Android in the market. But, I guess I'm wrong as the market spoke...
It wasn't even "the market" that spoke so much as US telecom companies. ("Vote with your wallet just means the rich and the mega-corporations have more votes.") Windows Phone had a good enough market share in Europe and Asia to remain a viable and competitive third place contender for a long time. It was the US where Windows Phone got locked out of the market duopoly by bad deals with AT&T and Verizon, and indications existed that all of the US telecoms were much happier with a duopoly than training/sales/marketing anything beyond that (whether or not that was an anti-competitive trust is left for your own imagination, it's not like the US has strong anti-trust teeth right now).
What you are describing is the usual problem of establishing a two sided market. You need users to make app developers care. You need apps to get users in the first place. From what I remember, MS already had to build some apps themselves / offer significant financial incentives [0]. This seemed hardly sustainable and thus the death of the product was, while disappointing, not a surprise.
So yeah, you don't need a conspiracy to explain this failure.
You can't get users for a product no one can buy. Calling it a conspiracy was largely a gag, but if anything it was a conspiracy of dunces.
Verizon refusing to sell and refusing to allow on their network the Lumia 950 because of a hissy fit that they didn't like how the previous flagship performed and that Microsoft got an almost favorable AT&T deal for the 950 was dumb on several levels.
AT&T getting bored with their 950 deal and then refusing to advertise/market/sell the phone, was certainly a death nail, partly because it was so much easier and cheaper to just micro-manage the Android platform.
It might not have killed the platform in the US if there were more than one phone manufacturer in Windows Phone at that point in time.
(That calls into question if the Nokia buy out was the right move. Which with Nokia last one standing already, it was probably the only move, but the platform had enough market share before Nokia was at risk of tanking that had a couple Android manufacturers gotten fed up with Google at the time things could have gone differently. Though admittedly, armchair quarterbacking is easier with hindsight.)
The app situation was always something that could have been addressed if people were (capable of) buying the platform. The lack of OEM manufacturers and the lack of support/interest from the carriers certainly mattered more than apps at crashing marketshare of the platform below the critical threshold for active application development.
I don't think people were ever going to buy in to the platform. I liked Windows Phone, but it had basically no substantial feature that made people think "wow, it's worth it to drop Android and iOS for /this/."
You got Android if you overwhelmingly wanted the customization. You got an iPhone if you overwhelmingly wanted something that was smooth and worked well. I tried WP for a while and I missed nothing major by switching away.
When Windows Phone died and I switched to Android, the UI didn't feel as intuitive as the Windows one did. And of course it doesn't help that Google change the UI, in particular the settings, all.the.time.