Sorry for nitpicking since this wasn't your point at all (and the comment grew, and grew, and grew as I was typing it), but...
> LoL is a beautiful and engaging game, despite the basic concepts
I assume you haven't played much LoL, have you? I've been playing for 3 years and I still consider myself a newbie and not only mechanics wise but also (or even specially) game-knowledge wise.
140 champions with their own stats (and growth per level), each with at least 4 abilities (+ a passive, some of them taking paragraphs to describe), plus two summoner-spells (to choose out of ~10), each with its own cooldown that you have to track (not only for yourself, but also for the enemy). ~250 items that modify up to 16 different character stats both in flat and % form, many of those with passive and active abilities that modify game rules and interact in often unexpected ways. 19 different types of crowd-control effects that can combine with each other in the same ability. 2 different types of invisibility.
Frantically trying to maximize your gold and experience gains last-hitting minions, while trying to survive and maybe kill the enemy.
In a game with limited knowledge where you can't see most of the map and only have a trinket that gives vision for a very limited time (lack of vision and incomplete knowledge being a feature, sought by the game designers). 4 different types of trinkets, useful for very different situations.
Then there's the neutral monsters in the jungle. 2 types of neutral monster (in each jungle, 4 total) that give you stats for some time if you kill them. 5 different types of dragons that randomly spawn each game. The powerful herald and baron. And there's a guy whose sole reason to exist is to live in the jungle hidden from your vision and come to your lane and ruin your existence surprising you with a 1v2 skirmish.
All of this in a constant flux due to balance changes and new champions regularly introduced (currently ~6 each year) changing the meta-game (or even the game itself). Plus reworks of many old champions that make them even more complex.
And that's the bare minimum! Akin to knowing how chess pieces move. Even the top of the ladder games, where strategical and tactical thought is critical, are simple compared with the complex strategies that eSports teams can carry out.
LoL is probably the most complex game I've ever played. It's just hidden beneath a thin layer of simplicity ("kill, don't die, destroy the enemy nexus").
That is what made LoL successful and why I think it can't be compared to Flappy Bird's success.