The American battle rifles aren't a weakness, they serve their purpose well. The military should be focused on things like IED detection, preventing brain damage from concussive force, battlefield intelligence, social programs (hearts and minds)... the stuff we are having a hard time doing.
The M4 is accurate, but lacks the power of something like an AK-47, which hits harder and has a larger round. It's great for clearing a room or working in tight spaces, but it's not nearly the same as this.
Additionally, they are working VERY hard to improve IED detection, and in fact are making quite a bit of headway in that regard. The same goes for every other issue you raised.
There's no reason you can't improve offense while simultaneously improving defense.
Improving offence helps you improve defence. If you make the most dangerous weapons in the world, then you solely have the ability to figure out the best ways to defend against them.
The US will have a significant advantage in developing jamming systems against these types of rounds.
The US isn't afraid of some Afghan insurgent living in a cave from developing a countermeasure against these new munitions. They're afraid Russia or China will be capable of developing a countermeasure. The rounds are worthless, or potentially dangerous, if you can cause premature detonation.
