Of course rarity (i.e. supply, or 'short supply') is not alone what counts: it's the basic law of economics defining price that counts: DEMAND vs. supply (rarity). A watch can be ultra-rare, if nobody wants it, no demand, low price. Actually, I think that if watches are too rare, it's not good - because there is not enough written about them, not enough 'noise'/hype, and prices are low. You need a certain critical mass to get a hype and interest going. And then you need the demand side, demand higher than supply. It's the differential between the two that matters. If there are only 5 watches of one kind, but only 5 collectors want that watch, price is low. But if there are 5000 or even 10000 watches of one kind, but 10x so many collectors want it (see Daytona), price goes through the roof - although the watch is much less rare. Demand and condition are much more important than rarity, IMO.
That's why I think Rolex is doing so well - there are enough watches around for enough collectors to own them and talk about them in forums etc. That sparks interest of new collectors and you get a virtuous cycle. And then, one you have many interested, you get the special kick that out of the many, there is - by definition - only 1 in the world that is the one in best condition of all in the world, and prices go stratospheric. Rolex is hitting the sweet spot of enough critical mass, enough history, great quality.