I agree the bellweathers are a good idea to make it more manageable, but also we have to acknowledge that some watches come into and out of favour.
A good example (actually I was discussing it with a very old time Heuer guy, Jonathan S) is the Monza, which if my memory is correct, was not that valued as a Heuer model, now, perhaps with the help of Arno's backhistory, it is much more sought after. Similarly I can remember when Montreal's were considered too ungainly and could be picked up for much less than other models.

I'm not sure if there are models that have become unfashionable, I have to think a bit more about that one. Anyone have any ideas?