All you're saying is that FOSS developers targeting Linux might[0] be able to lean on the generosity and hard work of Linux distro package maintainers.

That's great, but if you're a FOSS (or free as in beer) app developer targeting Qt on macOS or Windows you're choice is basically between shipping a bleeding edge version of Qt 6.x that will almost certainly cause regressions for your users, or ship a version of Qt 5.15 that almost certainly has known security vulnerabilities.

As an app developer bumping minor Qt versions on my users isn't something I want to do every other month, since doing so requires a lot of regression testing and user feedback. Most of the time I just want to roll in critical bug fixes (security issues, crashers, etc).

[0] In reality shipping Qt apps for Linux is really horrible. As you noted every distro is using a different version, so the users of your app don't all get the same experience unless you go to a herculean effort to bundle your own Qt build.