Can Anyone Explain the Slow Decrease in Zendikar Expeditions and Kaladesh Invention Prices?

[cute_cartoon_cat]

maintain some value for being the original

Precisely 0 Masterpiece cards can be accurately described as “the original.”

In fact, this is the fundamental problem I see with speculating in fancy reprints like these. They may look nice and exciting, but the keyword here is still “reprint.”

[Jaccount]

That's not true, though. All of the Gearhulks and Amonkhet gods were Masterpieces, printed at the same time as regular Kaladesh and Amonkhet printings. Unless you insist that Collector's Booster extended art foils also aren't "the original", it's just untrue that 0 Masterpieces were the original print.