I think Blizzard had (and has) an excellent team of devs, but they weren’t amazingly better than the dev teams of DAOC, ESO, FFI?, EQU-whatever and several other popular MMO games. I think Blizzard got lucky.
I think the Blizzard devs chose a particular balance of features that worked very well. They chose them carefully, but it was still largely a guess as to “what works best”.
The result was amazing. It took WoW 5 years to reach 12 million players, but WoW got there. Vanilla WoW did that. Why is it surprising that Vanilla WoW is popular today. It still has that special lucky balance of features.
I played DAOC every day for 3 years. The day I tried WoW, I switched games and never looked back. There was nothing revolutionary. In fact WoW copied a TON of features that DAOC had. But playing WoW was 30% better than DAOC, in several ways. Not surprising, since the WoW devs could see the good and bad parts of the older game.