I can demonstrate from my own collection how suppressed subsonic calibers can sound nearly-silent. Or at least anything not remotely sounding like a gunshot. And to most people’s ears completely inaudible on the other side of a typical residential door.
There’s still a need for low-power and close quarters firearms where rifle caliber long arms are less than optimal. I’d consider VIP protection and sensitive area security (nuclear reactors, HE storage depots, flight line sentries, etc.) to be instances where the wieldiness and downrange effects of the SMG is far better than a shortened rifle caliber long arm.
Protecting a VIP entourage or a HVT building is important but not at the expense of innocent people hundreds of yards away. You might have heard about how the London police used to issue 5.56mm Hk G36C carbines to patrol officers and had to retract that idea after a few occasions in which they were used in responding to a terror attack. Turns out that firing 5.56 in one of the worlds densest metro areas is a recipe for hitting several things unintended.
Pistol vs Rifle chambering means the difference between a superficial .45 ACP dent in the sheet metal of a parked $165 million jet and a 5.56 hole that may ground that jet indefinitely until repairs are made.
This SMG assessment by the Army is likely more related to those factors than the battlegrounds scenario you have in mind.
Too bad the title states “conventional forces”....which means grunts and sappers and treadheads....not slicky boys or CID... they, as already mentioned, have boutique weapons.