I didn't change my position on this complaint. I said the same in the initial post. Changing my defaults is shitty behavior on the part of Microsoft, and as far as I know (which is admittedly limited), they do it with every application they make.
"Then you complain about being unable to Uninstaller it when it's uninstall process is like literally every other application."
Now...this is a lie.
It is not like literally every other application. It does not appear in the Programs and Features list with an uninstall option. I googled it just to make sure I wasn't misremembering since you seem so certain, but others have the same problem. And the solutions are unpleasant:
https://www.windowscentral.com/how-completely-remove-paint-3...
Are you going to try to tell me that using the command line and editing the registry is how you uninstall literally every other application?
I provided, I think, actionable information in my complaint above (that's not to suggest that Microsoft reads or cares what I think about Paint 3D, but other developers are here, and if it prevents even one from making an application that changes someone's defaults without permission or makes an application that can't be uninstalled using the usual channels, then I would consider my complaint productive).