> It's a feature to me that I can't accidentally download an (potentially hostile) app from a (potentially hostile) app store, but a bug.

I don't really understand this point. It is not like Android implementation of side load is insecure.

As a user, you need to explicitly enable it in settings. After Android 8 I think, each application that tries to install a application is blocked first and need an explicitly permission too. Even with all that, you still need to explicitly consent installation and upgrade of any sideloaded application. Nothing is automatic.