The problem with this though is that you have to keep five hundred sets of gear just to do anything. In FFXI, which had horizontal progression, you had to macro entire gearsets before single actions. My BST had to macro a charm set to boost charisma high enough to charm animals, which was as many pieces as i could get (12 i think, needed two macros) then switch back to a DPS set. Healers switched from a heal set to a rest set every time they took a knee. DPS had a haste build to build tp then a weaponskill build to maximize damage.

Having all this horizontal gear is only useful if you can switch it on demand, otherwise you end up getting bis anyways. While you had a lot of options, realistically you were limited to a BiS depending on your level of endgame involvement. It's not a panacea.