The net is not the last I feel either, it's just more popular than the others and more commonplace.
I believe that it probably is the last, for a long time (but not forever).
When Al Gore was still agressively promoting his Information Superhighway (officially called the NII, National Information Infrastructure), a bunch of people brought up the subject of whether the Internet would be "allowed" to operate over the NII. [*]
To me, this seemed like an absurd question. It's like asking, "If there was a new service to send packages between cities for one tenth the cost of the current carriers, would people use that to send christmas presents?"
Any publicly available and cost-effective means of transporting IP traffic will be used as part of the Internet.
The Internet will be the last public data network for the forseeable future because each new form of telecommunication will be absorbed into it (e.g., ISDN, Frame Relay, ATM, cable modems, ADSL, none of which were around when the Internet was invented). And new services are added to the Internet all the time. The World Wide Web is now one of the biggest uses of the Internet, and it hasn't been around that long.
The only way I can see the Internet getting replaced by something else is if it finally hits a limit where it won't scale to provide a new service that is very much in demand. And I can't imagine what kind of service that would be. But that's the nature of innovation.
Note that I'm not saying that the Internet 25 or 50 years from now will necessarily much resemble what we have now, except in a general sense. What I am saying, though, is that there won't be a conscious decision made at some point to "start over" with something different. Even the much-ballyhooed "Internet 2" is simply a high-speed Internet backbone that has tighter access control. But for all that, it is still part of the Internet, just as there are many other private backbones with access control (i.e., MCI/Worldcom won't carry packets on their backbone for customers of Joe's Internet Emporium, unless Joe's and MCI/W have a peering arrangement).
I expect that this kind of analysis is what led Microsoft to their "Embrace and Extend" policy regarding Internet protocols, rather than their earlier position of pushing proprietary protocols. They figured out that the Internet isn't going away.
[**] Or perhaps, as usual, they simply have too much money [***] to throw around, and can't figure out a useful way to spend it.
[***] What money? Our tax dollars, of course. Personally I'd rather keep an extra few cents rather than sending it to NIST for development of new protocols that aren't necessary. But we don't get that choice, do we?
Last updated February 28, 1999
Copyright 1999 Eric Smith