The difference is the amount of content in your typical MMO. Since it seems clear that Destiny can't produce their content (or won't, but let's be positive) at an accelerated rate to what they have, they can't deliver a suitable size of replacement content to justify knocking off old content completely. The end game is very barren, and if they can't provide new content to fill it up they should unlock old content with a level boost. With House of wolves, you had two raids, survival mode with six Raid-style boss fights (perhaps slightly less complex then Oryx/Crota/Atheon), Trials, and Iron Banner as endgame content. Now in Taken King you have Trials, and one Raid. Iron Banner maybe if the next time around ups the ante to endgame level rewards. We even see this in strikes, at the top tier playlist in House of Wolves, you had eight strikes. In Taken King, you have seven.