There's nothing unfair about a company making a product people desire and earning from it.
The mobile market, like any market really, is not a cakewalk to win, nor keep. If it were a matter of simply dumb luck and plenty of advertising it wouldn't have taken and be taking so long for the traditional publishers to play catch up in this space.
Supercell's success can be traced to marketing, but not the incredibly limited understanding of what marketing means that seems pervasive. Marketing is about understanding consumer needs and from that formulating and executing a marketing mix that outperforms competitors in meeting those needs. And Supercell seemingly understand this market better than most.
They've wide released three games, and they've all been sustained successes.
And if they're dethroned it will be because someone else has met those needs even better.