There are two ways RL cards can lose value: bad economy and rampant counterfeiting.

If the economy gets so bad that even the 1%ers are tempted to sell off their collections, the cards will quickly become worthless. Everyone would be looking to sell and no one looking to buy. A great depression or natural catastrophe can cause this. Pretty much everyone is screwed in such a situation, so I doubt it's worth planing your life around.

Counterfeiting might hurt Magic as a whole in that newer generations of players will not appreciate the value of the cardboard. There will be no confidence in a card being the original printing, so fewer people would be willing to spend big bucks on them.

Eventually, people will be buried with their duals and Lotuses, removing them from the market for eternity. There might be a still be a Black Lotus for sale for +$1,000,000 but no one is buying because they're all dead.