That actually is a very good point. Video games differ from baseball cards in one special way. Video games are first and foremost meant to be played. They serve an actual function that most people want to take advantage of. Because of that, the market for the uber quality sealed games will be smaller. And if enough people aren't willing to pay $150 for a mint mint mint grade A+ sealed copy of Super Mario 64 then the prices won't be able to sustain themselves.
But people must take heed: Remember what happened to Marvel comics (and the comic industry as a whole during the 90s). People started paying attention to how Amazing Fantasy #15 and Action Comics #1 were selling for thousands and every Tom, Dick, and Harry became half-assed speculators. Comic sales went up...a lot. People were buying variant covers and all that crap. So production runs were steadily increasing. All of a sudden people realized how stupid they were being and how little money they'd actually make in the long run and so stopped buying. Now comic publishers were printing a crazy amount of books that nobody was buying and Marvel almost went bankrupt.
Imagine what would happen if tomorrow a good lot of the gaming community went out and started buying up every Atlus title they could find. Atlus would be thrilled...and would print mountains of copies...until everyone realizes how little these games will be worth and Atlus ends up sitting on piles of merchandise they can't sell.