A lot of what Blizz does is iterative, but for they are limited in iteration by expansion/patch schedules. Take dailies for example:
A LOT of people were annoyed with MoP's daily quest system, so Blizz changed it to the apexis dailies in WoD.
That wasn't well received so they changed it to a mixed daily/bar system in Tanaan. This went over better.
Legion they leaned hard into world quests and emissaries but we also had the Suramar storyline.
When Broken Shore came out they amended those with more classic style dailies. Argus went even harder in this direction.
One of the biggest problems with BfA is that it didn't change much at first. The initial patch felt just like legion. WQ and Emissaries.
Nazjatar and Mechagon then changed it up. Naz had the character based daily quest from your companion. Mechagon went back to a more old school approach.
Daily world content has changed a decent amount over the last few expansions but it has been done in small steps. If you compare Nazjatar to apexis dailies from WoD it feels way better, but that isn't the comparison a human brain makes. You compare this patch to last patch and see only slight changes. And honestly one of the biggest issues with that is the fact that you are still having to deal with last patch as well. Nazjatar and Mechagon may have been great, but we a lot of us were still doing wq in Voldun.