Consider declining T levels an adaptation to a society moving past needing ooga-booga aggressive males regardless of cause. Does not seem like a bad thing per se.
Has it not? it seems like there's pretty clear consensus that they are positively correlated.
Proof that T causes high aggression in isolation, in humans, thus far, has not been decisive. There is no consensus on this. Looking at the absence of the inverse (low T decreases aggression) would tip a few things.