When i started we had a mix of MP5s (which i love and miss dearly) P90s and 416s. All smgs were replaced with the MP7 at some stage, for no logical reason in my opinion. Just settling on the short 416 like we do now would've made more sense.
As for 300blk...
When they were relatively new we had a couple of Sig MCX rifles in 300blk, they were total pieces of shit. Extremely inconsistent accuracy and reliability issues, both with several different types of decent quality ammo (black hills, Ruag, Sig). Our colleagues from the close protection team for government officials have had Daniel Defense pdw rifles in 300blk for a few years now and had a lot of issues with those as well. They are running fine now and i know DD is usually putting out good stuff but still...
So that's our personal experience with anything that was at the time available here in 300blk. I know that's not the calibers fault, and i know there's good rifles on the market now and it's a good chambering for LE applications.
But we also know the 416s are working flawlessly for us for over a decade now and there's no need to take a risk and possibly waste more money again on something that might not be 100% good to go.
edit: if HK wasn't so damn slow in adopting new things a 416 in 300blk would've been a nice choice. But they barely now realized that MLOK handguards are indeed a thing people would like to have
edit2: as for 5.7, i have no personal experience, but heard first hand from operators from two different units in Belgium (one police one military) that had very disappointing results when using their P90s in defensive shootings/combat.