Hard to say, but for the most part it seems that Blizz simply has no idea how to construct a proper narrative, let alone how properly pace it.
The habit of discarding entire plotlines the moment the next patch comes out really isn’t helping things either; it already happened with large chunks of the leveling content and the nerubians in TWW, and the goblins will be shuffled aside whenever 11.2 comes out.
Being frank, Undermine feels pretty “disconnected” from the whole TWW narrative. The only links we have on Gallywix exploiting the Black Blood as just a special sauce to make weapons stronger (seriously, it does nothing else in the storyline) and Floatyfeet stupidly giving the MacGuffin-Battery to him for repairs - only for the shadow-mummies to randomly steal it because Floatyfeet and the Greedyjowls have no concept of an IFF protocol.
The Undermine storyline could, and largely does, exist entirely separate from those obvious tie-ins to TWW’s overarching plot. It was obviously built separately then shoe-horned in.
Supposedly, the mention was that Metzen was brought in too late to “fix up” TWW’s narrative (but not too late to do the overall announcement). So there’s a chance things will be different for the next expansion.
… but given the history of Blizz’ storytelling as a whole, it’s FAR behind the curve, and TWW isn’t doing them any favours. They ran out of goodwill two whole expansions ago, they can’t really say “it’ll be better with the next expansion” anymore - they need results.
NOW.
And FFXIV will be the perennial counterpoint to that.
Aside from the delay between patches and expansions, the pacing there is great. And what is included within a single patch (as well as the expansion launches themselves) is at least a satisfying experience.
No weekly time-gating, ever. The closest thing to that is the optional reputation grinds for the tribal/society quests, which is a set number of daily quests between each milestone quest. Just about everything else is released on patch day, maybe a week or two delay for the harder difficulty version of content… and the in-universe explanation for those are they’re usually “embellished retellings” of the canonical baseline difficulty.
There’s definitely some challenges to storytelling in MMOs, but Blizz is definitely making it worse with their handling of it.