So you would argue that heavy and armored is good if the role requires it; and fast and unarmored is better in other circumstances.
Just like the sticklebacks.
what did those planes evolve from? seems that the design of light armor and heavy armor both can serve the purpose that they were designed to do.
light armor heavy armor can be beneficial, but it already has the genetic info programed into it. now if you could show a fish Genus that never ever had armor to develop it you would have a better case.
Of course though I wouldn't argue thast specialization is always a plus.