Shack
That's the best argument against the US-2 but I'm not sure it's a disqualifying one. The cargo door / ramp capability is perhaps a requirement for a mission that may have gone by or one that we will not do in the continuous spectrum of conflict / competition with China and other challengers in other arenas.
The air / amphibious delivery of cargo (mass, wheeled vehicles, etc...) may not be what this platform could / should be doing for support in the Indo-Pacific, versus smaller, lighter, on demand deliveries and support of cargo, pax & fires / effects to outposts and teams sustaining, taking, defending and securing objectives (islands, ships, platforms, etc).
Taking a look at what the competition is doing, namely the AG600, it has no cargo door and only conventional crew / pax entry doors
It appears they are not planning on using this as a means of rapid delivery of outsized cargo to remote locations principally accessible via amphibious ops. As the most likely aggressor in the Indo-Pacific, if they don't see the need to have an amphibious aircraft with a cargo door, we as the most likely defender probably don't either.
As our Marine Corps (the principal land warfighter in the Pacific) is moving to a lighter, agile, lower footprint force structure, we as the supporting services should probably OT&E a certain small percentage of our force for that. If they intend to be unencumbered by heavy armor, fighting vehicles and the like then they should be supported by force that supports the light, small and agile.
Concur with your point on SOCOM, they like to modify iron not wholesale acquire it themselves (the fleet of platform X). I have no solution to that fact, only the conventional force providers have that much money and wherewithal to buy, sustain and operate an entire MDS, SOCOM ain't doing that so you have to convince the USAF, USN, USMC that it is (amphibious capable air mobility / utility platform) is in their wheelhouse. Maybe AFSOC would see this as an opportunity as the ME AOR is downsized in DoD engagement, not sure, not an AFSOC staff / braintrust guy but maybe...
For me, one platform, is interesting but it needs to part of an overall warfighting strategy for the air platforms of the USAF, USN and USMC, for the USAF as a part of ACE.
An amphibious mobility / utility platform, a manned multi-role manned tactical expeditionary / dispersed ops capable platform, an unmanned modular expeditionary / dispersed ops platform and a family of systems for logistics and C2 for these systems. A Cactus Air Force that can survive and move, fighting and supporting while under long range fires to its fixed bases and operating sites.
ACE is great and moving in the right direction but there is only so far you can go with systems designed and built decades ago with certain parameters and expectations.