Flarum's permission system being so complex/powerful, it's very difficult for Flarum to tell whether a user will have permission to create discussions or not after login.
Even when the user only has very few tags in which they can post, the button is still there for guests as a way to login. So it actually makes more sense for Flarum to always keep that button as a primary call to action on the homepage.
Because of that reason, it's probably best for an extension to take care of hiding the button if the owner thinks it makes more sense that way. If you want to disable the button instead of hiding it, you could take a look at the source code of the Readonly Profile extension, it disables multiple of the buttons in Flarum's UI based on a setting value. But the setting is tied to disabling login so you can't easily use it without forking the extension.