Here’s an explanation for lag from Blizz support, but supplied to us by a player. We can’t take it as fact unless Blizz give us the info. Are they lying? On the other hand, your statements are also not based on facts provided by Blizz.
The game is 15 years old, but retail and classic (aka legion) tech is not 15 years old. And servers are apparently not an issue according to Ion. What he says in Forbes relates to classic, but it makes sense that lack of servers are not a limiting factor in performance.
“That’s why we’re not jumping to, hey, let’s just open up dozens and dozens of servers. We have the ability to do that. This isn’t about limiting costs or available hardware or anything like that. We could do that easily if we wanted to, actually much more easily than having complicated tech like layering.”
After brief searches on Google, you see posts about things like Discord causing WoW lag. There’s probably lots of causes. Have people that experienced lag done everything recommended to fix it? Or just said, it’s the servers, and left it at that.