I can't imagine the US Army material procurement people accepting a bullpup design, but the features of the Steyr AUG need to be looked at, even though it has it's own shortcomings. You're right in that the only way to get a full length barrel in a compact maneuverable package for CQB and rapid response to/from motor vehicle transport is to adopt the bullpup style. If they're going to go that far, then they might as well adopt Steyr's instant-change barrel feature as well.
I've seen AR15-breed rifles offering 'Quick change barrels', but all of these need time and tools to accomplish at the armory where the Steyr AUG can be changed immediately in the field from the standard 20" rifleman barrel to a shorty 14" commando barrel or to a 24" squad marksman barrel.
Downside of bullpup rifles is the trigger pull and length of pull off the non-adjustable clamshell-style stock not comfortably fitting soldiers of various sizes, though the last issue is distinctly an American military characteristic: The Austrians have Tyrolian mountain climbing soldiers who are all pretty much two meters tall and don't have to worry about how their AUG length of pull fits a 5'4" Latina soldier from New Mexico.
There have been a lot of folks who've never really examined the AUG that describe it as the ugliest rifle they've ever seen, but once you give a doubter the chance to use it themselves they're quickly convinced that Steyr-Mannlicher was at least a half century ahead of their time when they developed the StG77 'AUG' rifle.
A long time ago, one of the most frequently cited objections was the shortened sight radius. Optical sights have made that a non-issue.