That's not what horizontal progression is at all. Horizontal progression as most of us want it would have gearsets that are variable based on the situation. The whole point of it is that there isn't one ideal BiS slot. What is BiS changes based upon the encounter or party makeup. People who want things like skill trees aren't really asking for horizontal progression, they're asking for more variability in ways to build their character.
As the game is now, though, fundamentally proper horizontal progression is impossible, so I believe that we need a balance. As I always do, I point to TERA for a vertical progression MMO that does a decent job of at least offering variable stats and equipment that change based off of the situation.