Well of course the previously dominant branch of philosophy declined- that happens all the time in philosophy. But I don't think that there's grounds for proclaiming Hellenist philosophy to be significantly better than its successors: it was hardly empirical (Hypatia herself was an anti-empirical Platonist) and typically more concerned with e.g. confused explanations of the world in terms of a single property (all is fire! no, water!) or confusion regarding words (e.g. the Sorites paradox) than any kind of research valuable/relevant today.
And the group which continued the legacy of Hellenist/Roman thought, the Islamic world, did in fact continue and, IMHO, vastly augment the level of empirical thought; for example, it's widely believed that the inventor of the Scientific Method was an Arab scientist, Alhazen. Even though Europe saw a drop in learning due to the collapse of the unsustainable centralized Roman economy and the resulting wars and deurbanization, all that occurred was that its knowledge was passed onto new civilizations large/wealthy/secure enough to support science/math/philosophy. (Specifically, Persia and Byzantium, and later the Caliphates.)
The technological and empirical tradition of Islam pretty much died out due to the success of The Incoherence of the Philosophers though. My point is that innovative and empirical traditions have given way in the past to memetically stronger anti-innovative traditions. That doesn't mean that the same will happen to present day scientific culture, I highly doubt that would happen without some sort of catastrophic Black Swan event, but innovative traditions have not historically consistently beaten out non innovative ones.
http://vimeo.com/22099396
What do people think of this, from a Bayesian perspective?
It is a talk given to the Oxford Transhumanists. Their previous speaker was Eliezer Yudkowsky. Audio version and past talks here: http://groupspaces.com/oxfordtranshumanists/pages/past-talks