The Army has it right with that medium ruck to compliment the MOLLE system. It does create a logistical issue I could see for the guys at Quantico to purchase a 3 pack system and most "make do" Marines can do just fine with what we have and be happy the Marine Corps got us something new.
I've slowly become convinced that the majority of infantry Marines do not need a large ruck. For what we tend to use the large MOLLE/ILBE/FILBE for most of the time, a seabag or kitbag with reinforced handles (and maybe a bit of compression) would suffice. I've said as much in the other big thread about redefining 2nd and 3rd line load carriage, but now I'm more certain that we only need tw o packs--assault and a 45 liter-sized setup.
The March Up was admittedly a mech fight for a number of reasons that might not hold true the next time, but the procurement folks need to look at what we did with the majority of the MOLLE rucks; straight onto lash-down points on the tracks, LAVs, and trucks. Once they were put there, they pretty much stayed there and even the Jansport bags saw limited use as well for whatever reason (maybe they were mostly filled with pogey bait
We have got to adhere to our own doctrine, and follow the most likely situation where we'll orient on the urban centers in the littorals, and the LOCs that get us to them. We don't need a patrol/72 hr/3-day pack that's optimized for alpine terrain and sub-zero climates, or feels like a pillow on our back at the sake of durability. It needs to integrate comfortably with the current and future iterations of individual armor (while optimized to carry a oddball item or two like a radio) to reduce wear fatigue and increase individual comfort, and carry a defined weight limit.
If you're a Reconnaissance Man and need to hump the heavy loads, you should have a system optimized for your needs, in much the same way the battalions maintain wet suits, fins, etc. There isn't a need to use the supply system to maintain a model. Use unit funds to procure the pack that work best. It might even be a modernized large ALICE.
If you're a mortarman, you can use a ALICE pack and frame that requires very little modification to optimize for use.
If you're a sniper, your unit's funds can set you up with anything optimized for operations, much like a 0321.
If you're a radio operator or a JTAC, the basic patrol pack a regular infantryman should have avaable will already be built to accomplish your needs with radio carriage.
Javelin gunner?...yeah, it sucks no matter how you slice it.
The average infantryman would do just fine with a two pack system. The rest of the mental effort should be applied to working the logistics to support the infantry fight, even if it turns out to by wholly dismounted.