I don't think I've ever met a single person using subsonic 556. Subsonic 22LR? Every day. But not 556. 223? Only when hunting deer. Not for social work.

Most rifle rounds depend on speed for their energy.

A subsonic 5.56 is only going to have 15-20% the muzzle energy of a standard round at best.

The ammunition is, but the mechanical parts of the gun are still fairly loud.

You can get the sound down to maybe 100 dBA. That's much quieter than the 160ish that you get without a suppressor. But, that's still loud. Things that are around 100 dBA include blenders, electric drills, motorcycles, etc. It's only a very brief sound, but it's loud for that moment. The Guinness record for loudest finger snap is 108 dBA. So, the quietest gun will be much louder than the average finger snap.

That means those movie scenes where people are having a shootout with silencers in a library and nobody notices, or even in a crowded train station. That's not realistic. People might not recognize the sound as a gun, but they'd definitely be looking around for the source of that snapping sound. It also means the scenes where someone is sneaking through a house at night killing people without being noticed are not realistic either. If you're asleep, a loud finger snap might not wake you up, but if you're awake you're definitely going to notice it.

It depends on what you shoot, also. A subsonic 22lr will sound like an airgun. A 55gr 5.56 (common to AR-15s) will still be loud as shit from the sonic boom.

Compared to...?

It's extremely quiet compared to shooting a gun without a silencer. Sure,, but guns are incredibly loud.

Agree. A 9mm with a nice suppressor? All I can hear is the “action” of the gun: Click. Click. Click.

Right now, the market for them is distorted by the tax stamp. Only a few people bother with the paperwork and fee (even though it's been highly streamlined in recent years). That means companies producing them have to make up their costs with high prices rather than volume. It's almost a stealth tax on top of the stated tax.

That results in only a few well off people getting them. This has little overlap with skill or appreciation for the hobby.

Drop them from the NFA, and now everyone with a 3D printer can just run one off. Even in traditional manufacturing, you have a much larger customer base, and the company can sell on volume rather than high prices.