WoW was a juggernaut in the MMO market back then and is still popular now. If WoW continued it's exclusivity we don't know how popular it would be today. Ultimately that argument doesn't really matter though.
I think attacking LoL for a lack of exclusivity and essentially calling Morello a hypocrite is a flawed argument. LoL's game design has never been about exclusivity. WoW's game design heavily revolved around exclusivity for many years and then abruptly changed. That's why I think comparing exclusivity with LoL and WoW is apples to oranges.
I'm not saying if Morello was in charge of WoW's development that it would be perfect; it could have been awful. But I think his opinion is a valid one. If you don't then that's fine too. But changing the argument from what he said about WoW to his game design in LoL is only deflecting and not a compelling argument.