In the current component dispatcher implement, if a request doesn't specify a controller, the dispatcher will use COMPONENT_NAME as default controller https://github.com/joomla/joomla-cms/blob/4.0-dev/libraries/src/Cms/Dispatcher/Dispatcher.php#L131. The default controller, in most case, just contains display method to display a view in the component.

While it will be fine for com_content (because Content controller is not use yet), there would be problem with most of other components like com_banners, com_contact, com_categories... because with these components, COMPONENT_NAME is already used for different purpose and could not be default controller, see https://github.com/joomla/joomla-cms/blob/4.0-dev/administrator/components/com_banners/controllers/banners.php , https://github.com/joomla/joomla-cms/blob/4.0-dev/administrator/components/com_categories/controllers/categories.php#L19...

With that said, we will need to use a different name for default controller to avoid this conflict. Maybe use Display or Default as default controller name will work