B71, your key assumption is that the role hasn't changed. The whole AO of the ADF has shifted. Basing a need on a few trips to some remote corners of PNG to qualify the Caribou replacement is outdated. The Chinook can do that short rage small airfield stuff better than a 'bou.
The C-27J mission is very different. Transiting from TVL to DRW in a Caribou is an all day affair with no payload to speak of is where the Spartan beats the 'bou hands down. It is also equipped for ops in A/Stan or other likely hot spots.
You may recall the Caribou operations with the UN in the Kashmir last century. The aircraft could barely clear some of the mountains up there without any real payload.
Like the airlines moving on from the DC-3 it is time to move on from the wonderful Caribou...