WAR is, hands down, the single best comprehensive metric of total value there is. It's king until someone devises a better quantifier.
Bryce won the ROTY on the strength of his whole season. He had the best WAR of any rookie in the NL. He needed that September to get there, but if he'd have posted that month in July instead, he would have still deserved it.
Maybe you could argue for giving the award to Cabrera based on Trout's slumping last month if the difference in their year long value wasn't so striking. But it was not close between them.
Cabrera just doesn't have a good case unless you over value the triple crown.