I agree with you that the iOS 10 is optimized for the iPhone 7, but we cannot forget that the TESTS were done with the old iPhones (6S, 6, 5S and 5C) and they had a LOT of time to do it (were many beta versions).
I said that they are ignoring the problem because they are simply giving 'tips' to save battery on iOS (like turn off the notifications, background refresh, updates, effects, etc) It is ridiculous!!
They should at least inform the users that an update will come to fix this problem. But they are not talking about it. (I talked to them on Twitter more than 10 times and I do not have obtained this answer).
My iPhone 6S for example, is new! (only 5 months of use).
And no, the update (beta) 10.1 is NOT solving the problem, not yet!