Direct comparisons across genres are very difficulty to defend, especially ones with wholly different monetization structures.
MMORPG’s by nature require a large time commitment to experience even a fraction of what they offer. It’s going to be easy for newer gamers to get into nearly any other genre besides an MMORPG, especially when considering delayed gratification is rarer today than it was in 2004.
League of Legends is free to play without a massive time commitment. Players don’t have to fork out real money to experience the majority of League of Legends while basically all new players would for WoW (unless a new player can figure out how to farm nearly 300K gold around level 20).
League of Legends is 100% a PvP game when compared to the mostly PvE nature of most MMORPG titles. We would need to have data on how competitive most people looking to pick up new games are in recent years to determine how much that might play a factor in players choosing League.
MOBAs are far easier to watch as someone new to the game than an MMORPG. This means players who are going to Twitch to watch someone game and then pick up the thing they’re playing are more likely going to pick up a MOBA rather than an MMORPG.
Blizzard has certainly made their mistakes over the years. I am not trying to defend their decisions here. But at the end of the day, WoW remains the number 1 played MMORPG in history and is still the most populated MMORPG on earth. Comparing to a title in a different genre introduces many unrelated factors that will directly impact popularity that it’s not a fair comparison; somehow I suspect you already knew that…
There is just no way you can have the data to know why retail player counts ebb and flow the way they do. Perhaps it is changes players find bad, certainly there have been changes I’ve thought were mistakes over the years. But it could also just be a genre that was never popular (relative to other genres) catching fire for a decade (likely in large part due to WoW) and then fizzling out as those who played it evolved and the new crop of players looking for something different. There are still professional photographers that use film cameras; are those people simply more tolerant of a theoretically worse medium for storing visual data?