Garth -
I'm not sure what you consider "steady use", BUT.......
I have well over 10,000 miles on D/A 8 spd STI's with no signs of deterioration. I did have to replace the cable on the RH side (rear drlr) not long ago and anticipate replacing the cable housings in the next year or two. Pretty comparable to any downtube shifter set-up in terms of longevity so far.
I have both a 'dale 3.0 and a Klein Quantum (the 3.0 12 yrs old and the Quantum around 10 yrs), neither showing any signs of age whatsoever. The Quantum is a polished frame, no clear coat, so I can inspect to my paranoia's content. No news is good news, and so far, no news.
Collectability? Well, the 'dale is the equivalent of a mid-model Chevy, but the Quantum is an early, low volume frame - so it approaches "collectability".
But, as has been said, the really collectable stuff is the interesting limited production stuff, especially team issue equipment.
- Sid, raining in northern Chicago land, Smith