I agree that Usman Khawaja was discarded before he was given a proper chance... he is such a talented classy batsman (in my opinion he is test quality).
When Australia were the best team in the world, they never seemed to have a bad batting pick (there were a few seamers who got a game when perhaps they shouldn't). The reason for that was that there was only ever one slot up for grabs - number six - and the new guy came in there and was bedded in before the order was rearranged however slightly to suit the next new man in. Even the guys who didn't go on to have long careers - Martyn Love and Stuey Law spring to mind - were given that chance.
Note how England mimicked this for about five years.
For Khawaja, you could just as easily read Root, Bairstow, Hughes, Quiney, Taylor et al ad nauseum. In, then out, then in again but in a different slot then oops, dropped, then back to open. With very few exceptions, this is a strategy that always fails - ask Ramps and Greame Hick!