Birdsong and Brajko are not better domestiques than Agnoli and Vanotti. They're better riders, yes, but doesn't necessarily translates to being a better domestique. Add that they're clearly out of shape, unhealthy in Brajko's case, probably not completely committed to helping Nibbles anyway.
And it's not like you need a train to beat Sky's train. It's unbeatable. Different strategies are required. I'm perfectly happy with Tira and Kessi, I'm sure both will prove very helpful for the aggressive tactics Astana will need to derail the train.