The question is: Why would anybody prefer a web-app over a native app in any kind of system or on any kind of device?

I think the answer is "only when there is no native app for the system I use", ie Linux.

So FOSS people want for apps to become much worse for everybody else, so that they can have the apps also through a web browser. Remembering that everybody else is who pays for the apps and all development, while FOSS people will never pay a dime to software developers.