Horizontal progression is not progression. It's being stuck in the same place forever. Vertical progression works because that carrot at the end of the stick is so damn tasty. In horizontal progression, the optimal builds become evident real fast. Any carrot that doesn't improve on the optimal build will not be chased. Eventually you will have the fully optimal build and know that barring changes in game balance you will never be able to improve. And then you get bored and quit.