Or you know, I understand the lifecycle of esports/videogames more than you do?
The biggest and longest ever esport in history was SC:Broodwar (see http://en.wikipedia.org/wiki/Professional_StarCraft:_Brood_War_competition).
League of Legends hasn't made as big of an impact on South Korea (the literal home of esports) as Broodwar did, and is already starting to lose players to other games who have managed to keep ontop of improving their game, while Riot as stagnated and focused more on milking the cash cow rather than improving their game for the majority of their player base, and for the professional scene (Turns out, people are pretty bored of seeing the same 10/15 champions banned/picked every game.)
League of Legends isn't like WoW, where people will play it no matter what because it's the best game in the genre by far due to its development team - because it's not. Valve and Blizzard have shown us that their MoBAs are simply more interesting. Because HoTS is a young game that statement isn't so true, but the fact that it is even capable of competing with LoL after just leaving Alpha is saying something.
As an esport, league will start to decline in 2 or so years time easily, as a game itself, it has already started to decline as more and more people are turning to other MoBAs and realising the grass IS greener on the other side.
Why? you're probably going to ask. Here's a couple of points:-
Boring champion design compared to other games (If you don't believe me, take a look at a couple of DoTA champions and compare, same goes with HoTS)
Summoners Rift 2.0 was underwhelming, and not the refreshment the game needed in order to stay relevant in comparison to DoTA and HoTS (Which has about 7 maps in rotation currently, with different objectives and play styles on each)
Severely dated client, and COMPLETELY lackluster/botched Macintosh Client compared to its competitors.
STILL has server problems in North Americas east coast and EUW still isn't as stable as it should be. Also: Packet loss.
Increasingly toxic community just because ATM it is THE most popular game, and every immature 13 year old with an ego has come from CoD to play LoL.