This could go on forever. But just a few things in the back of my mind.
A. If your primary weapon is sufficient (M4) why carry the extra weight of a pistol and pistol ammo, I would rather have more 223 ammo instead. Now if your weapon is going to fail, then it's time to get a better primary weapon.
B. The 9mm (355) is just a 38 Spl (357) dressed up to be used in a semi-auto, if it was judged insufficient for battle over 100 years ago what idiot at the DOD thought it would be OK now? Right use 17 bullets instead of 6, now there is a cost savings. Or did we change to keep our allys happy, they couldn't fright their way out of a paper bag, we have to come to their aid time after time after time after time.
C. If they were issuing Glock G-18s (full Auto) then I would consider the 9mm but otherwise a Uzi would be considered also.
D. Why don't we just get rid of both the M9 and M4 and go with a HK MP-7 in 4.6X30, easier to carry, more ammo can be carried and is more effective that either the 5.56 or 9mm. Very good for urban warfare. For long range just use a AR-10 or M-14.
Just my $0.02.
Jim