To me the mistake of open source software is to make programs compatible with proprietary operating systems, this way people that use these OS benefit from the open software, while people that use open operating systems cannot benefit from proprietary software that usually is exclusively for Windows or macOS.

Beside that, this adds a lot of work to the open source community, work that could have been better invested focusing on open source and not wasting time ensuring compatibility with the latest Windows or macOS release working around crap and restrictions that these companies impose to distribute the programs in their platform (e.g. all the signing requirements, paying fees in case of Apple app store, etc.)

Not that the user of open source software cannot run it on iOS or Windows, he can, but it has to port it there themself, no official support from the developers shall be ensured. Just distribute binaries for Linux and Android.