You're comparing these records to someone who wasn't included in this series because he wasn't deemed to be in the best XI though. Marsh has had a good series and made meaningful contributions in two different tests, without which Australia would have been in a pretty bad spot. He's been the third best batsman in the side, and really not too far behind second, which is Renshaw. 0 chance of Renshaw being dropped to me.
You might be right that Khawaja will come back in automatically for the home summer, and I think it's true that Khawaja is first XI at home and not first XI in India, but I think he'll need to make some runs beforehand to reclaim his place over Marsh as the incumbent. I simply don't see the selectors picking Marsh, who has a good record over the last 3 or so years, for a huge series in India, him doing well, and then dropping him for Khawaja because he's believed to be worse in home conditions. Keep in mind that Marsh missed out in the home summer largely due to injury. He was picked for the first test of the summer, did fine and then got injured. When he was fit in India he was picked again, and once again has done well. Khawaja on the other hand was omitted.
Above all I don't think the selectors and team will be happy with the idea of picking different people for different countries consistently. Maybe for one opposition or one surface, like adding a second spinner, but not just "we're back at home now so Khawaja comes back in". I think they'll need some sort of form-based reasoning to change the team, especially if that team has been winning.
