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.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.