I think the question of “what do they carry most of the time” Is a bit distracting.
My Land Cruiser spends most of its time in a shopping car park, but I’ve got it for river crossings and adverse tracks.
If it’s deemed to be unsuitable for these dodgey swamp holes, If I haven’t gone bush for 2yrs, then all it ever does is shopping runs, then sure buy a Camry.
I think the C-27s intended role was for ‘Tactical’ air lift, akin to Caribou utility.
That actual Caribou-esque role/requirement still exists, especially with the pivot to dispersed littoral operations and the logistical support of it.
Part of the issue is whether or not the C-27 is really effective as a replacement for the Caribou. My suspicion is that the answer to that question is, "no."
IIRC, part of the original reason why the US Army wanted the C-27 was that they wanted a smaller airlifter to work with and augment the C-130. The idea took some hold because it turned out after studying some of the logistics flights, many of those conducted by C-130's were only flown at partial load capacity. Again, IIRC the RAAF at one point did a similar study and reported similar data, with many RAAF C-130 flights being largely or mostly empty aircraft. Hence some of the push towards a capable airlifter with many of the capabilities of the C-130 in terms of range, size of load, rough field performance, etc. The end going being to get another airlifter which would cost less to operate and would be suitable for many of the missions conducted by the C-130.
Unfort lobbying by the USAF got the DOD to turn the C-27 battlefield airlifter project over to them, and the USAF not wanting to lose control over fixed wing airlift transports promptly killed it.
Of course the C-27 programme itself was not without some flaws and issues, since whilst not as expensive per flight as the C-130, it was still not a low cost aircraft to operate. In the end, I believe that the USAF decided that the (apart from the political machinations) slightly lower aircraft flight cost was outweighed by the increased costs associated with operating a second type of aircraft. This was even after commonality in avionics and mechanical systems between the C-27 and C-130J was achieved.
Getting back to the ADF generally and the RAAF specifically, aside from questions regarding the size, weight and type of payload being lifted (pallets, outsized cargo, personnel, etc.) I would focus my questions more on areas where the ADF wants and needs to operate fixed wing airlift from, and whether or not there is a real difference in the number of fields suitable for the C-27 vs. the C-130.
If the situation is that any field the RAAF would use a C-27 from could also use a C-130 then trying to sustain to different fleets might not make sense. OTOH if there are fields which could take a C-27 but not a C-130, I would be interested to know just how many of these there are, and how often they are used or needed.
As I mentioned before, based off public data available for the C-27 and DHC-4 Caribou, there are likely quite a few fields that the Caribou used to be operated from, but are too small and/or the field surface is too soft to take the much heavier C-27. I seem to recall someone here on DT who was ex-Army, mentioning that during an exercise a group of troops was used to manually lift, turn and reposition a Caribou that had landed in the Far North somewhere so that it could take off again. It would likely be quite difficult for a unit of troops to even try and do something similar with a C-27.
