2 Points:
- MS has been making cloud elements available to iOS devices for at least 2 years; I think the MS focus is facilitating interface with their cloud (Azure) environment instead of getting involved in the OS culture wars.
- I bought a Lumia 735 a few years ago because I needed a W10 mobile capable device that would be available when the OS was released. I was able to upgrade from 8 to 10 without a problem and have been able to update as well. I have stuck with it through dying apps and the end-of-service announcements and intend to go forward with it until I can't. My Lumia provides consistent map service, Outlook mail and calendar service that syncs with my PC, and the best WiFi tethering I have ever used working with both my Win10 laptop and my iOS 12.2 ipad. Also, and important to me, a replaceable battery instead of a sealed case. I have no reason to think that I will have any reason to drop my Lumia unless my carrier (Verizon) does something awful in that regard.
Best wishes - Go Forth & Conquer