You pay 12.99 eur/month to have access to the game and whatever meta the developers feel like supporting at the moment. If Sub Rogue and Ret Paladin aren't doing what you need, you have 34 other specs to choose from.
It genuinely baffles me that people still get upset over PVP balance in this game. It's been 17 years. The game has never been balanced and likely never will be. Either play something you enjoy, or play something that's on meta. That's all that really can be done at this point. Blizzard has a terrible track record otherwise.
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Requires a core gameplay redesign. Not of the spec, but of the whole game. Demo and Affliction have different gameplay mechanics, so it's not really a matter of "scaling up" damage. If the scale Demonology up to be competitive with current fight design, the instant the decide to make a true tank and spank with genuinely minimal movement, Demo will skyrocket. The problem is game design, not spec design. And if Blizzard has shown anything, it's that with each expansion, they're going to throw more and more bullshit mechanics at us to prevent us from doing damage.