Development of military technology without the world wars

[MattII]

SMGs might be good for rear echelon / support, the nice thing about them is they use the same round as your pistols (or they should), a PDW might mean introducing a new cartridge into your supply chain just for that weapon. Also PDW's and body armour are kind of linked and we're a ways away from commonly deployed body armour

SMGs/PDWs would also be good for vehicle (both aircraft and ground vehicles) crews. And the former doesn't even need to concern themselves with muddling ammunition types, since they won't be carrying any other ammunition types.