People don't want yet another close sourced OS like iOS, they want something open like Android. Microsoft has the resources to work with other bigger service providers. Also a lot of corporations use Microsoft's solutions like Outlook and Office 365. They could provide a strong focus on corporate customers sorta like how Blackberry had that reputation. Point is, we got two options for smart phones, we need more.

Who are these people? What is the size of that market?

The fact is that iPhones and iOS dominate the revenue of the mobile market[1]. iPhone users have incredibly high customer satisfaction numbers[2] and are unlikely to switch, so essentially you are competing against Android which is already "open". Seems like an easy way to loose a ton of money to me.

[1] https://www.zdnet.com/article/how-the-iphone-x-drives-apples...

[2] https://marketrealist.com/2017/06/could-apples-iphone-8-driv...

[3] https://www.imore.com/apple-seeing-record-rate-android-users...

>People don't want yet another close sourced OS like iOS, they want something open like Android

So you've conducted a poll and asked what people want? People don't care if the OS is open or closed source. In fact, the extreme majority wouldn't even know what you're talking about. They care about having access to the apps and services they want. Microsoft failed with a closed source OS and they'll fail even harder with an open one too.