Of course Paul is the expert, but I would add that it is very interesting question because what I always notice about the VSL instruments is the evolution of the library as compared to the original sound quality. I have all the original library, and all of the newer. One can trace a growing mastery of refining the sampling process, so that no redundancies are found, and in the later instruments - such as the phenomenal Bass Flute, 2nd Clarinet, or of course the Dimension instruments - a perfection that is not found elsewhere. However, the original instruments are still - to this day - some of the best sounds of all the sampled instruments. In fact, some of the old ones have MORE samples, and more variety of samples, than newer ones. This is exemplified by the near-maniacal level of sampling found in the solo C trumpet, tenor trombone and horn, which have a HUGE number of articulations and variations. (And which also caused physical injury to at least one player during recording due to this thoroughness.) In addition, the ensemble instruments, which are among the oldest, and have relatively fewer samples, are actually some of my favorites. I cannot function in society without the ensemble 3 trombones, 4 horns, 3 trumpets, as well as the woodwinds. And this does not bring up any of the strings, which include the oldest - the Orchestral - which have the most complete articulations in terms of variety of all the string libraries and are indispensible for any string performance. But at the same time, the programming of all these original instruments have been updated, so that many tweaks and additions have been added.
So the point is, the age of some of the instruments attests to the lasting value of their incredible performance /recording quality, which in the oldest ones has become like vintage wine. One can never improve upon simple supreme musical quality.