Haven't we basically already built them? They're just slow and not supported by software vendors so nobody wants to use them. Other than replacing capacitors and realtime clock batteries on every 20 years or so, dusting and replacing fans when bearings go bad (assuming it's not a passively-cooled design), most computers should basically last beyond a human lifetime (I've read that those less than ~20nm will go bad over time as traces lose atoms and eventually fail, but older processes should be fine).