Actually, GW deliberatly IMbalances armies. Then "improves" them by cycling those imbalances through the various codexes, insuring people will buy more product to stay up-to-date or just to keep up...or simply to have whatever is the best army at the time.
Remember, the purpose of 40K is to sell minis. Balance and stagnation sells little to none by comparison to throwing things around as often as they can. In an ideal world for GW, you're thinking of buying new models every time a new codex comes along, regardless of whether it's actually for the army you like playing.