I see no particular reason to transition away from 9mm. When restricted to FMJ loadings, the difference in unarmored-target performance between 9mm, .40 S&W, and .45 ACP is not great enough, IMHO, to offset the advantages of capacity, light recoil, and NATO conformity inherent to the 9mm. While 5.7x28 may have an advantage in the penetration of soft body armor, adversaries who even bother to wear body armor at all seem to be smart and well-funded enough to opt for hard armor that will stop most centerfire rifle cartridges, much less a handgun bullet.
I do, however, think that, even when restricted to FMJ, that a much better 9mm loading could be devised than the current-issue M882 loading (112gr RNFMJ at approximately 1263fps). Personally, I think something along the lines of a FMJ-FP at 1200+fps or even a 100gr FMJ-FP at 1400+fps would be a superior loading to the M882.
Smith, and Wesson, and Me. -H. Callahan
Well waddaya know, one buwwet weft! -E. Fudd
All bad precedents begin as justifiable measures. -J. Caesar