Much of what you pay for these days has nothing to do with the optics. You are paying for AF motors and stabilisation technology on top of the glass. Nor is the age of an optical formula is necessarily an indication of its quality, particularly with prime lenses (there are some classics from the 60s and 70s that can still hold their own today's glass). Zooms are a slightly different story but I doubt if the best zoom from 10 years ago produces visibly inferior images to the best zoom today. You also get product variation within certain tolerances.
I'm sure you are correct. I understand the wider availability and relatively cheap cost of zoom lens today is almost entirely down to advances in the manufacturing processes. It's not 'better glass' but, as you say, more or less the same glass with autofocus and stabilisation manufactured cheaply for today's mass-market.
