Even if you’re beating players with higher level cards, it doesn’t mean your skill rating should endlessly go up – for that to happen, you would need to beat all of your opponents much more often than you lose. For all players (except the very best 2-3 players in the game, perhaps) this can only go on for so long until the player finds their (current!) sweet spot and they plateau.
I’m not sure if that makes sense, but basically everyone has some “skill” level. Generally, this goes up a bit as players play the event and tweaks their decks and strategies. Players fall into a few broad buckets: some can beat players with better (or much better) cards, some players find it tough against those of equal level, and some struggle against those with lower level cards. Every player is given (roughly) increasingly harder opponents until their win-loss ratio becomes less strong (and again, this happens to every player eventually). The goal is for players to win about half their matches (it has to be this way, since every match has a winner and a loser!) … so we show harder opponents until this is roughly the case. When a player reaches this win rate (roughly), they’ll also have reached roughly their skill rating (and shortly thereafter, medal) plateau.
Regardless, every player will be able to raise the skill rating to whatever level actually reflects their current skill (indirectly impacted by the cards you have, of course) … and once you hit that plateau (regardless of which bucket you fall in) your medals will also plateau.
Of course, if you find a new strategy and start winning more often than not again, then you’ll resume climbing and reach a new, higher plateau.
But everyone has a plateau, and will eventually reach it. If this wasn’t the case, then winning the event would be about who plays the most matches, not who has the greatest skill (a combination of strategy and, of course, card levels).