For instance, we stuck an old TV capture card in one and now use it as a television replacement and media centre with a stripped down version of Fedora 14 with Blackbox using XdTV for television playing and Gnome MPlayer for media playing (accessing videos we have on the rest of the network through sshfs). Assuming we ever did loose support for the graphics hardware it has (Radeon 9200) and just stuck to the latest version of Fedora that works with it, I do not see how that would be putting us in that risky a situation.
Another box we are turning into a custom router and server box with ClearOS, but there it does not really matter what graphics hardware works or not, or indeed would not be particularly dependant on much custom drivers, so we can use the latest versions on it. On another we installed Windows 98 to play some really old Windows and DOS games we have still lying around, but if something happened to that machine it would be of no loss. Being a Windows machine it is pretty much isolated from the rest of the network anyway.
We have a really old and great Fujitsu Lifebook which runs Windows 98 on it as well at the moment, but if I were to put something like the Fedora LXDE spin on it would not really matter what graphics or display drivers it has since it has no acceleration on it anyway. In the end the only really place where the lack of security updates really matters is on the case of some newer laptops made about ten to eight years ago that still may be carted around. But these are for the most part covered by the radeon drivers or Nvidia anyway, so they do not seem to be under threat.
This is not deflecting the point, just something that I have been considering.
And the other answer is we should all give a big hand to those that are already doing this.