“the Secret Service uses the FN P90 in 5.7 to protect tbe President.”
Which is full auto.
The P90 is designed from the bullet up to defeat soft body armor by making a lot of small holes very fast. The FiveseveN handgun and related bolt action rifle are intended to complement that capability.
Alas, full auto is denied us peons. Marvelous as the 5.7mm is (a well improvement over the .22LR), and as excited as I am seeing the 50 round mag on an AR platform, it just isn’t worth it if full auto isn’t available.
You can get ammo for the 5.7 up to 3300 fps. This allows for cavitation on part with almost any gun you can name, in a light bullet with almost no recoil. Full auto or not makes little difference; the round is what makes these Guns effective. Legacy rounds simply cannot hold a candle to them.
Now you're selling speculation as knowledge.
Both the 5.7 and the 4.6 were specifically developed to supersede the 9mm in all specifics. That a full auto PDW was first in production is completely incidental. Just look up the NATO request or check Wikipedia.