His career is following a similar trajectory to Jonathan Trott's, who averaged 50 after 43 tests but was already on the decline from averaging up in the 60s at one point to eventually just 44 when his test career ended 9 tests later.Ohh ****, my bad on that. Didn't include Neutral venues. That comparison is probably fair given he was a 30 average shield bat at that point, but you can make the same argument for kohli and Kane early career.
FYI, I agree with you on the whole, I think he'll end up at 46-48 average batsmen by the time he's done.
Not saying Marnus will be done in a few tests time and Trott obviously had issues to contend with that Marnus doesn't (as far as we know) but even quite a short poor run can significantly dent a test average after only 40 tests.