Outside of Smith, Warner and Labuschagne no ones averaged much more than 40 for Australia for around five years.I think they've explicitly said Harris will open.
Personally I would have gone Usman opening and Head at 5. Harris I think is one of those guys ala Handscomb whos technique is good enough for domestic but not international level.
You could probably put Head in the same basket but I think it's more excusable in someone who bats at 5, not to mention he already averages 40 at test level.
Averaging 40 obviously doesn’t mean much as Head and Khawaja have both been dropped despite doing so.