I have to disagree with this bit. When I was in the Infantry we used to lug our assault rifles down very small, claustrophobic tunnels that were just wide and tall enough for your average Soldier wearing Battle Order to crawl down them in FIBUA/CQB training (MOUT/CQB for the American Servicemen). We didn't like it, but we could do it. Basically the issue comes down to concealability what with the easy ways to solve assault rifle length and size issues in close environments (C7A2 with its 6 position stock comes to mind, when I was in we would just rest the stock over our shoulder for room clearing). Of course the "six-poz" was adopted mainly for fitting in and out of APC doors and to compensate for the thickness of body armour, but Infantry always find at least three uses for each bit 'o' kit, or else it's considered "under utilized".

As far as the SMG istself being less or more, sure you could hide it and it would be a bit faster into action from "stand easy", but really the big plus is the fact that the current batch of pistol calibre SMGs apparently have better penetration and stability than the super short barrelled (10.5 inch and less) ARs used in their stead. Regardless, I am not volunteering to stand in front of either!

SMGs do have a role to play, just more for police/CT/Covert work than battlefield applications.

Outdoors, gunfire is loud and will screw up your hearing over time. Inside a building all gunfire is painfully loud. Assault rifles are much louder, but after the first few rounds of gunfire nobody is hearing anything for a while. With assault rifles you are more likely to get seriously screwed up permanently, but they are all bad.

Smart shadowrunners invest in low profile ear protection that they can leave in all the time. Like these: Electronic Earplugs

The guy I know who uses these walks around shooters all day without any issues outdoors, but does add muffs indoors as the indoor sound is so much louder that he's worried about long term damage when running classes through shoot houses.

With one of those special cases being the need for sound suppression. A truly suppressed rifle firing subsonic 5.56x45mm ammo will be pushing a ~62 grain projectile at around 1000fps. Compare that to conventional M855 ammunition, pushing a 62 grain projectile at about 3000fps. At that low velocity, the projectile is not very likely to fragment at all, which will result in a very narrow permanent wound cavity.

On the flip side, try suppressing a .45 ACP SMG, like the HK UMP. Your 230grain ammuntion will still only have about 990fps of muzzle velocity out of a 8" barrel...that's still in the subsonic range. Since the projectile is already subsonic, you don't need to use velocity controlled ammunition when fitting the UMP with a can. Quiet as can be, and quite lethal against unarmored targets. And still a good bit shorter than an M4 w/ a can.

There is still a significant niche for SMGs, moreso in the shadows. And especially if you need to be quiet.

As far as I know, nobody expects assault rifles suppressors to be be really silent. The marketing literature is mostly focused on reducing the potential for hearing damage and getting more effective communication due to not having to scream. As a secondary effect they also make it much harder to locate the shooter by sound or muzzle flash. They are designed to fire normal ammo.

I'm not sure if you could get a standard AR to run on subsonic ammo, that's a huge change to a very complex harmonic system. If you wanted that I'd suggest silencing a 22 rifle, because that's about what you have.

But I think the USMC also keeps some of the colt 9mm suppressed AR-15s around for those sorts of jobs, but they were found to be pretty useless past 100 meters.