Beltre was needed, but I found that he was more common on the market which makes me believe there were a lot of unlocked cards out there compared to other cards needed for collections. Purely anecdotal though. When flipping cards there was always more cards being bought/sold on Beltre than other "collection" cards I flipped.
But all the other points still stand. A card moving from diamond to gold or gold to silver increases their supply and lowers their price. You're basically saying what I said. Ortiz is more common now so his price drops, more people can afford him, but still need Pedroia. Pedroia is still scarce since he is a gold still, but more people want him now that Ortiz is cheaper and more common.
I'd argue Cano tanked hard because he became almost useless compared to other 2B options (Kipnis offers better offense vs RHP for instance) and a lot of people already did the Mariners collection since it wasn't that hard to do. Beltre is still a solid 3B option if you don't have Brett or Schmidt. I think player usefulness also comes into play a bit in cases like this.