Fan service is not a denigration. It's often very good to give your fans what they want. The problem is that WoD was cut short in order to put all resources into Legion, resulting in an overall weaker product for WoD. Legion was good, but it used up everything left in the tank, resulting in the trainwreck of BfA.
Fanservice itself is not inherently bad, but the way in which Blizzard used it was. They used it to cover up weak and bad game design decisions. The random drop of legendaries causing massive performance imbalance. The AP grind. The layered RNG of the Netherlight crucible.
There were some very good parts about legion also, but in retrospect the fanservice helped grease a lot of squeaky wheels. You can see the difference in BfA, where there are also some good things(equal to Legion, imo), but the bad sticks out MUCH more because the fanservice is nowhere near the same level.
That leaves us going into Shadowlands with...what? There's no big bad that we know of to fight. No legion, or old god, or existing character besides maybe Sylvanus. And if they turn her into a raid boss, people are going to have a bad reaction because they're sick of fighting their own faction leaders(even though some schadenfreude will occur due to the sylvanus haters). You can see Blizz scraping the bottom of the barrel with "The Great Unpruning". Trying to fob players off by bringing back old 'iconic' abilities. But what else is there, really?
I don't know. I just look at Shadowlands and all I can see is it falling short. Obviously I'm at the far end of the pessimistic scale(as any of the blizz-clan fanatics will gladly tell you). But being objective as I can be, I just see Blizzard being creatively bankrupt at this point. They've got nothing left.