Being able to select more units would actually influence the game balance, craclings would be more effective. UI isn't just a cosmetic change, the UI changes made to SC2 had changed how the game is played. In SC:BW you had to focus much more on the macro, you had to constantly go back to your base, you had to know when to do it (you should be doing it even when there is fight going on you just need to know/decide when you can do it, and leave your units without supervision for few seconds), in SC2 this exist to much lesser extent. Also even better AI of units (pathfinding) would break the game, this is the reason why there is so much flanking in sc:bw, that hardly exist in sc2.