Some UWP apps offer mod extensibility (Edge extensions are the most prominent, but a couple UWP games have community mods and more are supposed to happen down the road), and Microsoft has shown interest in supporting general UWP application modding/patching, but haven't yet solved how they want to support that (and still try for security/reliability concerns; not just let it be a new malware vector).

If you think its about malware vectors and not about them having greater control and say over apps and app distribution on windows, I have a bridge to sell you.

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.)

You could always patch the os to disable signature enforcement. That might open you up to malware though.