Perhaps a new case/motherboard design that lends itself to better scaling down the line would be in order. Again, keeping connections uniform, standardized, and backwards-compatable could make this possible.
I wouldn't care if it were a tube 5 feet high in the corner of the office if it meant you could use the chassis for MP upgrades for years to come. And yes, I know that in the short term that this would make less money than reselling computers every 2 years, but what a way to guarantee the platform's longevity.
And maybe the motherboard itself could be of a modular design, so that once bus speeds begin crippling the processor power one could simply load in a new one and put all of one's processors back in it, with a standard front and back panel for new connections (FireWire4, GigaWire 3, superduperultraSCSI 9, holographic lightpipe, etc.) so that the system itself is perpetually upgradable.
"Listen here, Sonny. Yer ol' grandad bought this Mac nigh unto 12 years ago, and while we've added three new motherboards and have gone through 6 processor additions and upgrades, she still keeps cookin' in that same ol' case with that old-school bulletproof power supply. We call 'er 'the ol' smokestack'--ain't she a beaut? Now let's go ahead and recalculate the movements of every subatomic particle in the universe, boy. And this time don't leave out the dark matter and antimatter particles, either... "