You seem to think it's an either/or situation. It's definitely both. It's most correctly that the two concerns are directly related: app distribution is the primary source of malware. The "classic" app distribution platforms for Windows, EXE installers and MSI are terrible for preventing malware. After decades of people complaining that Windows is the most malware-ridden platform, can you blame Microsoft for wanting to make app distribution better?

I can blame them for wanting to take control of the market, yes.

Sure, you can, just don't mind me rolling my eyes at you. For more than a year now it's been as easy, if not easier, to sideload an APPX on Windows 10 as it is to sideload an APK on Android. (The same ease from a user perspective as using an EXE installer or an MSI file: double click, navigate the security prompts.)