"metagame" does not equal "balance." I don't understand who you think should "tweak" the metagame... Relic? Relic tweaks balance. Players develop the metagame.
Thats partly right.
Players (most time the best and most experienced players) develop the metagame, correct. While doing so they tend to find the most efficient and easy way to beat there opponents because they want to win. So they check out the amount of options the game offers and lean into the options which support this objective at best.
And thats the problem. CoH lack´s options. Talking about 1 vs 1 there is some sort of balance but there are also just about 2 strategies and a lot of useless units. Relic has to tweak the balance but Relic also hast to tweak the game design itself. This will give players the opportunity to develop a more extensive metagame. And i am talking about options & strategies for the mainstream player, not just the top players as the always got more options (Twister talked about MGs for example) because of there better understanding of the game.
To sum it up, the metagame is made by the players as a result of options the game offers based on the game design and the current balance of the game.