Old Rolex in nice condition are collectible, but doesn't mean valuable. Lot of vintage around at reasonable prices.
The collectible are the sports models as they tend to be the most popular and the variations etc.
While a Datejust is popular. it will not hold its value as much as a Submariner model
Investment in Panerai today is a no go really. It was the early models - '93 logos, Mares, pre-v and early V models. After those. A very limited model that has meaning. 195 or 360
Tritium which is no longer used adds value. Something that is no longer available usually will increase some or a lot or not at all
Any watch will increase in value as long as the company is popular and they increase the prices of their watches which has a pull up effect on the previous models
If the prices from '98 were the same original retail as today. Watches would be low priced, but as retail prices rose. It increases the value of previous models (not including the earlier models which rose due to supply and demand.
My G series base logo is worth more for 3 reasons. 1) - price rose $1600? over the original cost, 2) The dial changed from L Swiss L to L Swiss made L, so only 500 of this model and 3) A number used this model to DLC coat reducing the number of original models