Odd that you pick out FFXI as a counterexample, as FFXI is probably one of the best examples where horizontal progression WORKED. Gear in that game lasted for YEARS, and yet people still kept playing because there was always other pieces of situational gear to obtain that would make their character just a little bit better in those situations.
While you are correct that certain jobs occasionally outshone others, that had zero to do with horizontal progression, and everything to do with SE making job adjustments that shifted the balance, or added new endgame bosses that favored one job over another (for example, NIN tanks were king until SE started adding bosses that regularly used horrendous attacks that ignored NIN's evasion-based damage mitigation).
Your bit about incorrect subjob is also perplexing. If you have the wrong subjob... you level the correct one instead. Just as in this game, you were free to level any and all jobs on a single character, and leveling a job high enough to use as a subjob was a relatively quick thing (I say relatively, because pretty much nothing is quick in FFXI).
At endgame in FFXI, you saw folks in a variety of different kinds of gear, depending on which pieces they'd been able to obtain. Players were not excluded from party because they did not have a specific gear build. It was understood that while every job had pieces of gear that were best-in-slot for specific situations, not everyone had the time or luck to obtain every scrap of BiS gear. There was just too much to choose from, and obtaining each piece tended to be a long-term endeavor.
In the end, FFXI, too, became a vertical progression game, with the introduction of gear ilvls with the Adoulin expansion. But for more than ten years, and over three full expansions they were able to maintain a stable playerbase and never had to increase players' job levels, thanks in large part to horizontal progression.