They were truly endless, there were always clear diminishing return points that made it obvious to non-obsessives that you could stop for now and until artifact knowledge caught up. The problem is that many obsessive people play these games and they will stop at nothing if a sliver of min-maxing can be extracted from a thing, ruining it in the process for everyone else.
Also, people do find vertical progression fun, that’s why they engage in it and seek gear/power. This is not a single-player game as you and others have pointed, rewards are the intrinsic part of the experience.
Why the HELL would you ever need to do everything Korthia-related? What useful armor did it provide that you couldn’t supplant through raiding and M+? This is exactly the kind of obsessiveness that lead to people spamming Maw of Souls 100 times a day.
I do agree that the seasonal model is predictable and boring because that’s what Blizzard has been doing, a new boring public event, a new raid if we’re lucky, and maybe a new dungeon once per expansion, but it’s not so much of a design direction as in the gameplay model that a vertical progression model devolves into.
What are the alternatives? Moving to a horizontal progression model would make a ton of people, me included, stop playing because I do find gear and character progression fun. If you have vertical prog and no seasons (or tiers as they were), then what are you doing but running the same content for 1-2 years?
Because getting gear is fun, that’s all there is to it. No one except status-seeking pushers and certain types of organized guilds goes after gear only because they need it to succeed, people want it because hitting harder and getting stronger is fun (which is then partly invalidated by scaling but it’s still happening).
Well yes but no, the reward IS the part of the equation and there is still a hell of a lot of difference between gameplay even among MMOs, that’s why the reward structure is so important. I do love how WoW plays even though I don’t do any activities that don’t have associated rewards, but I wouldn’t play FF14 and ESO because I can’t stand how they play for an example.