I'd say it's not so much about whether lenses can improve that much, but that the manufacturing technology isn't evolving in the same way. If someone could come up with a clever way to make it significantly easier to manufacture a lens then at first I'll bet that the third party manufacturers, Tamron, and Sigma, would start making cheaper lenses, then Canon and Nikon would eventually be forced to follow suit.
Yeah, a guy can dream.. last night I DID, actually.. I dreamed I had what appears to be the Canon 85mm F1.2 (a $1700 lens).. that'd be a pretty sweet Disney lens, although perhaps a bit too long for ride usage.
It was a curious lens to dream of because really I crave bigger, longer lenses. I'm fascinated with those ultra wide primes, but what I really want is an ultra long prime.