This doesn't hold water. You can always improve upon what's already out there. Hell, league grew to be the biggest game in the world in just a few years, and it was even beaten to the punch by HoN (as I recall) and obviously Dota itself, but it still pulled ahead and is now dominant.
I played WoW at a high level when it was in its prime, and I play league at a high level now in its prime, and in both cases the games had serious, glaring issues that knocked them down several pegs of enjoyment. For WoW, it was always class/gear imbalance ruining it, which always would've been insanely easy to fix with a few number tweaks and the disabling of a few blatantly problematic items, or even just doing balance patches more than once a decade. For league, it's solo Q. Solo Q just destroys peoples' enjoyment of the game. It may be the most successful game of all time, but literally every single one of those ~70 million players DESPISES solo Q. Together with the game's dogshit matchmaking system, the entire ladder is basically a joke and almost every game is an exercise in frustration due to teammates.
It seems to me like every time a developer releases a game, there are a couple of gigantic issues with it that just make you wonder how these people are even employed. GW2 was a perfect example of this. ArenaNet had a ton of people hyped for that game, and then... they release it with zero attention to balance, zero end-game content (and not even any type of relevant progression for pvp), and nothing even resembling a ladder, or even resembling competition at all? What the hell do you think is going to happen, the fucking thing was a ghost town within the month, and will never recover.
Any of these things could be vastly improved upon without a lot of thought or innovation (league's issues might be a little harder to fix than WoW's, admittedly) and would produce a game that is clearly better. Whether or not it would succeed is anyone's guess, but this crap we have to play right now is nowhere near the end-game for any of these genres.