Horizontal only truly works when the system realistically allows it. XI, for example, had appropriate horizontal progression, though in many cases it really was either a one go-to or macro'ing 2 different pieces for everything. It had in-combat gear switching, which is impossible here with the current system... that's a HUGE reason why horizontal gearing works. They can be used at any given moment whenever you see fit. If such a thing existed in XIV, the potential would be far more appropriate.
By example of where we are now, people will NOT bother with something that slightly gives you better potential output if it means lowering your ACC and risking misses throughout an entire fight if they can't switch their gear mid-fight. This means that it's a pointless addition to add in substitute pieces. In order for a need to change things to do that, there has to be substantial reason to allow it, meaning different stats will have to function differently than they do now. If you think the gear bloat is bad now, horizontal progression is significantly worse. For non casters, building TP meant adding in equips with Accuracy+, Dual Wield+, Double Attack+, etc, depending on what job and the other stats involved. To be honest, there is a form of horizontal progression already in XIV, but it's not based on BiS... it's based on getting there. Think progression gear when it came to Coils, people having to buy crafted combat gear to meld them with materia where it may be needed (ACC, VIT, etc). A situational equip, exactly as horizontal progression works. The process as a result, is not much different than the horizontal progression you refer to. It just falls short once you hit BiS, which is essentially what happens even in horizontal progression. Unless it goes to the extreme and requires dozens/hundreds of equips for a single class/job to perform in a BiS manner for every little task possible.
Note: I forgot about the destructive feature that is "ilvl". Wow did that concept destroy horizontal progression and promote a "must have X" mentality.