The civilian PDW, my Sig MPX Copperhead.

[B Cart]

but that seems such an extraordinarily niche role to have, to sacrifice everything else with the Copperhead, when something just a smidge bigger can have a lot more capability.

I agree with this. While i have some PCCs in 9mm, and wouldn't necessarily feel under-gunned with them, my main concealable PDW is a 300BLK with 7" barrel, PA EFAB brake, and micro dot. I run the Barnes 110gr Black tip in 20-round pmags, and it hits a lot harder than 9mm. Worth a little extra size IMO