> are alternatives
Are there? Really? No, they're not. No one who isn't in tech is going to install F-droid. Few people will uninstall untrusted sources, or even know what that is. Fewer Apple users than ever jailbreak their phones.
For over 90% of the market, no, there are no alternatives (other than a mobile website). Due to the non-standard nature of ARM and SoC/binary drivers, we don't even have a real alternative OS for old Android devices that's universal like most Linux distros on the PC in the 90s (although PostmarketOS is making some huge strides in this area).
30% is an insane cut. Bandcamp sells music with a 15% cut, literally half of Amazon/Google/Apple. I always prefer it because I know the band gets more money and I get a higher quality version of the song without a patent encumbered codec (FLAC and OGG downloads are supported).
But no, there are no real alternatives. AppStore/Play are bastardizations of the software repo concept from Linux.
If Google Play was a real package managers, it'd keep standardize shared jar files across apps (dependencies to reduce massive app size) either by mirroring maven repos and signing dependencies or creating their own system, and they'd also allow for easily adding 3rd party app trees.
Both Apple and Google are closed walled gardens, where everything you need has to be contained in your bloated package, and where their corporate overlords get final approval of what is in and out.