@AntoFlames : This is kind of a insolvable problem. There is one fact : players from the beta have an advantage over new players (those who don't pay). It's hard to say if the problem is that important or not. Time will tell if new players can dethrone us. Personally, I think that with good focus on upgrades, you can beat kings 2 levels higher than you. So the game is still playable for now. In the future ? I don't know.
By the way, you're level 10 and I'm... level 10. And you will never face level 15 players... because they don't exist. Actually, I guess the maximum is level 12, and high level players are mostly level 11.
And it's the interest of upgrading your cards (so your king) : you will be stronger for competition. And it gives an objective.