You have to think of it in terms of players. The thing is, fighting in only battle armor has to be a feasable strategy, as much as anything else. When you are fighting two or three other players, no matter what they are using, it makes sense that you should lose, on average. If this were a strategy game where not every unit was controlled by a person, then it might make sense for them to be rebalanced as you're suggesting. But even then, there are many, many instances in canon where a small group of elementals was able to defeat a single mech, of any weight category. In fact, sometimes the heavier the mech, the easier it is to down it. Many large expensive units are built for dealing heavy damage to a large target, and as such are actually quite weak to BA. Just like how you should be sure that your team has anti-air support, you should be ready for BAs as well. They are actually very easily countered. Most partisans will rip a whole team of BA to shreds.