Shadowlands is hardly the moment where the plot completely collapsed I would argue. It's crime really is just being kinda boring where Legion was fun in a camp way, and BfA was awful on an entirely new level.
The plot in WoW has never been great, or even really that good. The most consistently good writing we have gotten out of WoW was during Cata where the plot required no contrivances or leaps of logic to function. The plot just managed to happily chugged along without getting bogged down in trying to tell morally ambigious stories (BfA), breaking down fan favorite characters for the cool factor (MoP), or indeed changing several characters into villains jsut because that is easier than coming up with good reasons for it (TBC).
The crime of Shadowlands is really just that I really do get the impression noone really cares what is going to happen. The plotline with Uther and condemning Arthas is about the most engaged I am in the plot, and that is really just a side thing despite such a juicy source of pathos.
The main villain came out of absolutely nowhere in the most blatant way, and the story surrounding Sylvanas is more engaging to players on a meta level of whether Blizzard will redeem her or not, which isnt really interesting for plot reasons, but more so in being a litmus test for whether Blizzard has actually learned from previous failures regarding hastily redeemed characters.
BfA at least had story players were engaged in for the actual characters. Whether Saurfang would die honorably was not just intriguing for the meta level of writing where we care about the inherent writing quality, but also because it was a narrative hook with a lot of weight. The question wasnt just "Will the writers be willing to let Saurfang acknowledge the Horde being evil?", but further that if this happened, how would it pan out narratively?
Shadowlands really is just kinda boring. Much of this does definitely fall on the severe drought caused by Covid, but it also does fall on the plot really not beign that engaging. It has moments of quality, I still get shivers when I watch the Bastion cinematic and hear Arthas' last words, or when Uther feels regret over being unable to adhere to his teachings. The quest where you fight Tyrande and get that really good rendition of Nightsong is absolutely stellar in achieveing what it wants.
However, these moments are few and far between. And worse, these moments are barely linked to the main plot. The main stroy is currently about stopping a big blue man we have no clue about from finding a mystical thing we also have no idea about, which in turn will allow him to enact his dastardly plan to maybe one day actually tell us what his plan entails.
SL didnt necessarily need to be good, but it needed to be engaging. Legion managed to be engaging because it succsefully managed to ramp up the stakes to absurd levels while still maintaining a core focus on stuff that the players would actually care about. And BfA managed to sustain interest in the narrative through is absolutely atrocious writing for the same reason. Both those expansions focused on stuff that players genuinely care to see. And this is where Shadowlands failed.
tl;dr: Shadowlands is bad because it is just really, really boring. It has no relatable narrative core that players genuinely want to see through. And while BfA is infinitely worse in the writing department, it at least managed to be interesting, both because it had this narrative core to care about, as well as its writing being bad enough to warrant so bad its good status.