The iPod touch 6 has bad battery life so that's probably why. They would have to give it at least A10 just for better efficiency. In fact, that could be why it got A8 instead of A7.
I'm hoping the 5s/A7 gets an extra version of iOS just like the 3GS and 4s did. Then even if they drop the iPod with the A7 devices it'll at least get iOS 12.
Oh, yeah that is probably why. The iPT6 has the worst battery life on a current iOS device right now.
I don't feel like A7 will get an extra year though because of its GPU which on the Mini 2 and Air especially is having a hard time keeping up with the new animations. Even if the 5s runs better because of its smaller screen I feel like it would still have to be dropped b/c of the iPads.