But yes, capitalized "Internet" refers to the "Connected Internet," of which there is only one. The first rule of SIPRnet is no one talks about SIPRnet. But if we did talk about it, the comment would likely be that it is like "the Internet" but not "the Internet."
If you spend time in academia or academia-adjacent industrial research, you sometimes hear "an internet" (always with lower case to signal it's a common noun) to describe a network of networks. But you are right... that use is not growing and if anything shrinking.
Maybe some aliens out there have their own internet. But then is the internet just the sum of all intranets and thus in a way just the top-level intranet?
