It occurred to me that one reason great scholars of the past appear far smarter than they were is because generations of subsequent scholars have ironed out their work and added layers of supposed depth. A large part of this is the assumption that when a scholar contradicts himself, the solution is never that he changed his mind, or forgot what he had said possibly decades earlier, and certainly not that he made a mistake. Instead, the "apparent" contradiction is reconciled. So a taana's words accrete the clever insights of amoraim and geonim who lived hundreds of years later, rishonim who lived a thousand years later, and continues to have depth added by current achronim. These centuries of accreted cleverness contribute to the perceived greatness of the taana, who is assumed to have meant all the things that later scholars attribute to him. All of these insights are seen to reflect the scholarship and intellect of the taana, and he appears far more brilliant than he may have been.
So too with amoraim, geonim, etc., but each epoch of scholars has a couple of centuries less worth of clever commentary than the one before it. The older a source is, the more commentary and clever reconciliations is has accreted. So the older the source, the greater it seems, and we have a seeming confirmation of yeridos hadoros.