I think Anet's reasoning on forcing so much horizontal progression comes down to this:
"People aren't making diverse builds or learning the game's mechanics. Therefore, we need to do a better job of teaching them to people. Spreading them out over a longer period of time forces people to notice what they've gained and put it to action."
While this line of thinking isn't incorrect, I don't think it's the best solution. Quite frankly, the reason people don't tackle diverse builds is because the game doesn't require it. There's simply not enough feedback built into the game to really get people to shake up their builds.
"People aren't making diverse builds or learning the game's mechanics. Therefore, we need to do a better job of teaching them to people. Spreading them out over a longer period of time forces people to notice what they've gained and put it to action."
While this line of thinking isn't incorrect, I don't think it's the best solution. Quite frankly, the reason people don't tackle diverse builds is because the game doesn't require it. There's simply not enough feedback built into the game to really get people to shake up their builds.