VPN will be taken for granted some day, probably transparently, and probably sooner than later. Many of the currently popular network services are based on protocols that simply cannot be secured at the packet filtering level. When all is said and done, I suppose Linux security issues won't change all that much. New security problems will be uncovered in every piece of new software, and every time existing software is modified. So much of what is involved in securing a system is at the level of what services you choose to run on your Internet-connected machines, and how careful you are when configuring those services.
Nothing can protect someone from their own bad system administration choices.
Perimeter security technology has come a long way, and the general public is increasingly aware of the need for security precautions. I suppose the current trend toward backdoor exploits, viruses, and macros will continue to become an ever-increasing problem for the next few years. UNIX systems aren't particularly vulnerable to those kinds of things, but the systems UNIX firewalls protect are vulnerable, especially Windows systems. The Internet as it is today is a lot like our stereotypes of the Wild West. The Internet environment is a whole lot scarier and more dangerous than I ever stressed in the book.