I think that the argument that rock X, which is however many millions of years old, is better than rock Y, which is however many millions of years plus a few decades old, is pretty redundant. If there is a difference in quality then I think that it would be due to being able to be more selective a few decades ago, when the supply may have seemed more inexhaustible, as opposed to today, when we are becoming more aware just how finite these resources are, and rocks that may have been discarded back then, now make the grade to be sold. But, as always, we are dealing with natural articles that do not have the consistency of synthetic abrasives. Each hone is an individual, and therefore it is equally likely that the one in a million hone could be found today, just like they could have been found decades ago.
Another factor at play here, and one that is not just limited to coticules, is that today there isn't the same diversity when it comes to sources of honestones. Considering the coticules, Ardennes Coticule have worked one mine, and have recently reopened another. Going back to the 1800's Belgium was exporting hones from many areas, including but not limited to: Ambleve, Bihain, Herbeumont, Lierreux, Namur, Recht, Salm-Chateau, Spa and Vielsalm. With more areas exporting more hones, competition was higher and only the best would have been able to complete in the international market.
I have coticules from: Ambleve, Bihain, Herbeumont, Namur, Spa and some that are labelled as Old Rock. There are definite differences between these hones, as would be expected based on the fairly wide geographical range, but they aren't all excellent by virtue of the fact that they are vintage. The hones that were mined near Spa were regarded as being without equal in the rest of Europe when the stones were being worked in the 1800's, and my Spa hones are better than my other Belgians. My Namur hones are excellent also, Namur was one of the principle sources of export for hones in the 1700's up until the mid 1800's before their supply was superceded by the more familiar sources.
Also, way back when, Coticules were used primarily as razor hones, and those that were exported would have been selected based on their suitability for that particular purpose. So that would explain why vintage coticules may seem better to us as razor enthusiasts. With the rebirth of the straight razor being a much more recent phenomenon, the Coticule has been adopted by other fields as well and therefore the selection of the stone more recently, would have to embrace all applications, and not just razor hones.
As long as you are buying from a knowledgable source, who knows what you intend to use the stone for, I don't think one has to worry about how much better the vintage ones may have been. Coticules today are being retailed by many people who are primarily straight razor enthusiasts themselves, and therefore know what a good coticule for a straight razor is. Older stones are not better simply because they are older, it's just that certain excellent sources of hones may have been worked out may years ago, and as the stones became more unavailable so they become the subject of myth.
Kindest regards,
Alex