People will do anything to justify cool SMGs like the MP5 (i understand! they are cool and in movies!). All the complaints about "muh ammo reliability" ignore that roller delayed blowbacks are very picky with loads and suppressors, and parts need to be changed when you switch ammo. On top of that, MP5s are infamous for being picky about hollowpoints. A modern direct impingement SBR will be noticeably more reliable in nearly every way with various loads/suppressors.
The MP5 is still a solid gun and nothing to complain about, but it's 100% obsolescent. The Honey Badger was designed solely to outdo the MP5 in every way, such as weight, length, size, range, power, accuracy, and capability. Price wise, they're close enough that it doesn't really matter in a military situation.
Ammo doesn't really matter that much, as if you can make 9mm or 5.56 cheap you could totally do that with .300 if you wanted to. Idk what the US military uses for ammo, but I'm sure that some countries produce their own .300 for mil/le use.
.300 dominates in the AR/MP5 body size category, but to make 9mm better you'd simply just use a more size efficient gun, such as the B&T MP9 or whatever else they make. Mag in grip is pretty much the best way to run 9, as why not save on space? I don't really see any other decent SMGs/PDWs being developed besides B&T. Probably because of how useful they are for average people, quantity needed for mil/LE, and hoops to jump through to even get one as a civilian.
10mm or a super hot .45 would be pretty sweet for an SMG, but that'd be a trade-off of gun vs gear capacity. Because of the diameter vs length of the cases, the .300 gets you better gun capacity but less on-body capacity because they're longer. Alternatively
The people that actually need and use SMGs and PDW/CQB guns seem to have their shit figured out and seem happy with it, using 9mm, shorty 5.56, .300/7.62, or 5.7/4.6. They all seem to kill people well enough, and past a point you're splitting hypothetical hairs.
