I am sure there are more but those are my reasons for the old box.  But fan
noise, power consumption, and time may easily tip the scales toward an
appliance solution

I'm interested that nobody's done an appliance package.  SOmeone somewhere has sky-high piles of second hand network cards and P200 systems.  Throw a specialised *BSD install in, sell it as a "ultimate home router" for $75 or $100, and you've gotten way more value of the machines than they'd get as desktop comps (I recall a local shops selling that sort of box, like six months or a year ago, at "3 for 100")