What’s a natural monopoly? Valve currently has the freedom to implement anything they want within an extent because they’re so popular. If they decided they wanted to charge devs 35% would people stop using it? Probably not. Steam’s monopoly is as bad as any other for the same reason any other monopoly is bad.

Those repackages of libreoffice packages are actually official.

https://www.libreoffice.org/download/libreoffice-from-microsoft-and-mac-app-stores/

These versions are free software (as in open source) but there’s a small charge to cover the effort of putting software in the app stores, and to help develop the software (and build our communities).