Balance gets thrown around too casually on many games. So it either means you want something to be better or you're actually talking about player investment/reward. Focusing primarily on balance isn't necessary to make a good game in fact, many don't focus on it particularly and are more engaging because they focus on the constantly evolving meta. If characters have more choices and they emphasize the old idea of mixing class features then the meta can evolve so there are several ways to do well and some things work better in different situations, rather than based on job selection. When you lower the skill ceiling on a game you alter the skill-luck axis so you're left with only low skill low luck (one or few optimal ways to play, same gear/skills, rotations) play and high luck low skill (grinding rng) play. That is one way to design a game but it isn't the only way.

However changing the entire game to be more freeform wouldn't work not because of balance but because you're changing the design of the Skinner box which is the safest bet for an mmo. The box is made more comfortable with cosmetics, engaging story, and fostering communities. But more importantly people grow used to it and if you change the fundamental game style, rather than some academic concern like balance, your concern is alienating the existing fans who already like it the way it is.

Having two sets of classes to play would also be a lot of work, for what? You can already (mostly) solo with the other classes. Then you create the question why BLU isn't a full class if other classes can be limited classes, and if BLU is a full class it has to be cookie cutter like the rest.