Hello,
Because the idea of override is not to do in such a way to be able to update Joomla and extensions without loosing your customization.
You only specified updating Joomla / extensions, so I was only trying to be helpful by saying:
If you only change our template files, updating Joomla! or any extensions will not affect the template files.
Nevertheless, can you explain to me how we could implement an override above our template overrides or show us an example of a template that provides this ability? This sounds like a great idea!