And the plugin authors would simply self-host them.
Some of the top plugins available (and are linted) on WordPress.org tend to be stubs containing a lite version plus an ad, and those plugins will have less friction if they entirely self host on their own servers.
Too many people doing that will mean that what you're looking for just won't be in the main (GPL?) plugin area. My own experience years ago was that for many folks, if it wasn't in there, it didn't exist. Downloading and FTPing 'out of store' plugins just wasn't something that folks wanted to get in to. Those projects may have been a minority of the overall username, but it wasn't an insignificant number of projects I knew about. Perhaps that attitude has changed in the last several years?
Some of the top plugins available (and are linted) on WordPress.org tend to be stubs containing a lite version plus an ad, and those plugins will have less friction if they entirely self host on their own servers.