The DOJ killed Windows Phone. If Microsoft wasn’t worried about antitrust suits, they would have given Windows away for free on phones to build up market share and OEMs wouldn’t have had to go to Google to get a free OS.
Though a lot of that was still momentum/inertia, too. Microsoft had always charged for Windows on every platform, that business model was largely sound and safe.
In Windows 10 there was enough push to change that business model, but the hesitancy in the Windows Phone 7 and Windows Phone 8 timeframes to do that probably did lead a lot of handset manufacturers to "free" Android SKUs.