I think the cards should only go up in value. They are all playable cards, and the ones that aren't printed any more can only be combined into fewer cards, making them more scarce, thus more valuable over time.
It's the DEC and the SPS I'm not so sure about. Sure, their price looks nice today, but after weeks and months of airdrops... I don't know. In any case, I think it's smart (for myself) to spend my DEC on cards - leveling up my Beta cards - before they get even more expensive.
That's a smart way to look at it as well. Keep the cards, add some cards, and be less careful about selling off some of the passively earned DEC.
SPS - is the total supply of it going to be airdropped? If yes it's 'scarce' as well, if not it might be wise to start selling a portion of it as well at some point.
It can't hurt to enjoy some of the riches either, right? :D Thanks for sharing your thoughts @stortebeker!