I was 'in' 10 years ago and from my deployments in the flatter of the two sandboxes and the the hills of east Asia, I firmly believe that the military is holding onto backpacks for tradition more than anything. I was in the Infantry btw so I have a lot of Kg's carried.

Without going in depth about this, I attribute my opinion to the fact that everyone is rocking body armor these days and few teams can take big packs out for extended patrols without choosing between armor or big packs or broken backs. Time O'Brians short story, 'The things they carried' still rings true. From memory Body armor with plates and flak vests ~8kg, then 0.5Kg for every Magazine (7-10), 2 kilos of water, Radio 0-1.5, helmet, nvg, rifle or gun, and then maybe a rocket or mine. And that's stuff outside of your pack! The military is going heavy with armor so vehicles become the pack and soldiers just orbit out of those. When I was leaving the military more and more folk were moving towards having a day pack around the 25-30L mark that would be good for about 36 hours.

Anyway the only person giving me orders these days is me, so I take my suggestions from the ultralight hiking crowd.