What incentives does a game like WoW give for running older content? I imagine things along those lines would work right?
Yeah. Star Wars does it (though with their matchmaking tool so you're forced with randoms for better or for worse). Also TOR changed the loot drops where you got the old outdated gear, materials needed to craft current tier gear and consumables, and gear that was one tier below the current tier to help out with current content.
Basically the stuff players get will help them get gear to assist with the current raid tier. Since Destiny's entering Year 2, we'll be hitting the second tier of endgame PvE content (first tier being VoG, Crota, and Skolas).
However, Destiny's weird since gear from PoE > gear from Crota > gear from VoG.
Perhaps Destiny is really broken up into three tiers of PvE content then, with TTK's raid being the fourth?
Never thought about that.