As we level up, we outgrow zones and instances. This is a pretty good idea. I'm not sure I'd enjoy it if the Barrens had kept leveling along with me, to the point that I was running Wailing Caverns at 60, or 85. We accept that instances and zones are designed for certain levels and leave them where they are.
Battlegrounds are different. Why? It doesn't make all that much sense. Balancing them is made far more difficult when the classes in them will have much different abilities, and potential gear levels, as they go higher. For example, Warsong Gulch used to be a really great place for a shaman in the 20s, back when we had ghost wolf and no one else had mounts or even druids had travel form. Later on in AB, rogues had the handy ability to sap one person at a flag and start the capture, seriously disrupting the enemy. Of course they might have failed by not having two people at the flag...
Level 19 Warsong Gulch, even without twinks, would not be the same as level 80 Warsong Gulch. It might be more or less fun. My own experience was that it was less fun. In contrast, AB seemed like more fun at higher levels.
In the past Blizzard expressed reluctance to add new battlegrounds because they didn't want to spread out players too much. That's a good thing to worry about. But why 'solve' the problem by not adding new BGs? Put level caps on the old ones and then new ones can be added without spreading out the players. AB might end at 69, WSG at 29 (joking, maybe), AV at 79. Then come the new places, with plenty of players available.
We might miss the old BGs. Hey, remember AV? And WSG and AB? Fun stuff, right? Sure sure. But now we have TAK, BD, and RKG! Those places are so much better. Weren't we a little tiny bit bored anyway? I mean, it had been the five millionth WSG game and ever since the last patch warlocks were an auto-win in that place (I am making this up, I have no clue of what warlocks are these days, except that they're not as awesome as they used to be, because nothing is or ever will be).
We might also miss Wailing Caverns (lol). So fine, go back there again and take a look around. There could be an honorless queue, where people could join old BGs and fight, but with no honor gains, just like defense in AB (zing, but I think they fixed that a little bit). Why no honor? Because we're trying to keep you in the new places, and because then it's just like a level 85 going to Wailing Caverns to farm some epic linen. Woo.
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