Before gvg, I was good at arena (somewhat). I usually hit 9 wins, and even got my first (and only) 12 win key.
Now if I reach 3 wins I'm surprised.
It usually goes this way: I play as best as I can, I keep in mind what I become vunerable to before making a play, I try to get as much value as I can without being greedy, I make decent drafts, I watch Trump playing arena a lot, etc. Yet somehow, I always end up in a point where the board looks good on my side, but then the opponent goes in with a wombo combo to my face, and suddenly I can't win anymore because I don't have taunts/heal. I had a 1-3 rogue run in wich all my losses were like that.
I just don't know what happened, and it makes me want to give up on arena. But it's the most efficient way of grinding gold, and quests only take too long.
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They exist because they're cheap and simple deck to craft and play. Not everyone has every great card to build other decks so as long as there are effective decks that don't cost much to build...they will continue to be around.