If you are much better than the market at predicting how cards will trend, you should probably be working for Star City or some other secondary market giant.

Probably the continuous uptrend in the P9 et al. can be understood as rational if the continued growth of the game is uncertain. There's always the black swan possibility that Wizards will catastrophically fuck up in some way and hence let them tumble down. In addition, the growth of eternal formats is itself limited by the availability of staples. I would suspect there's an upper limit to how expensive the Moxen and friends can get on this basis alone - logarithmic growth of the game entails linear growth of Vintage and Legacy. This is, after all, why they created Modern, for which Modern Masters is possible.