And that’s ultimately the problem: The Internet isn’t a thing, it’s an activity. The Internet is, at base, a specific way to communicate data between different kinds of networks without needing (much) manual intervention at the network boundaries, as a result making those network boundaries seem to disappear most of the time. It has, in modern times, become the method, and the term, for pretty much everything we do as regards moving computer-readable data long distances. It wasn’t fated to happen, and it had to defeat the OSI Model and Protocol Suite in order to get where it is today, but that’s where it stands now.
So the question the OP’s really asking is what would happen if somehow, magically, the people of the world suddenly forgot how to Internet. And the answer to “Magic?” is always and only “More Magic!”