If this brawl rewarded packs from the new expansion (that you couldn't open until release of course) then I would be excited to do it

I'm sorry... but no, you wouldn't. You would look at the 1000 gold to enter and the amount of wins that is needed to just break even to come to the conclusion that you are not guaranteed, not even favoured to get anything out of this brawl that you couldn't get with 1000 gold.

The argument "This brawl gives me packs that I already have" is a mere excuse. It doesn't matter what the rewards are when you're so unlikely to get them. Ever stopped to consider that? The only people looking forward to play in this mode are the ones who don't need the packs and do it just for fun or to get more viewers on Twitch. That's it. This brawl was not meant for your average joe and I really doubt that streamers are doing it because it gives them gold and dust.

but I would rather have an unpredictable meta with a good reward system then a predictable meta with a bad reward system.

The reward system in this brawl is everything but good. It favours Blizzard heavily, even more so than some casino games do for their casinos (some people did the math back when this brawl was introduced). With an unrefined meta, even less people would attempt to play it. Just look at firebat, who recently went 12-2 with a jade druid teched to deal with Pirate warrior. He knew what he was going to face most likely and prepared himself before dropping 1000 G/ 10$. Now try doing that when NO ONE is sure of what works and what not. You wouldn't play it. Period.