One of the novel features of the P320 is the modularity.
It's a neat theory. But how about practice?
In theory; theory and practice are the same. In practice they're not.
The vast majority of M17's will be the full sized service size. I do not see the Army buying a complete set of grips, barrels, slides and magazines for every gun just to have the unused components languish in storage some place for at least a decade.
The theoretical savings from having "one" pistol that's several sizes only saves on the very margins.
Springs and magazines are the parts that get busted most, and those aren't the same between sizes, are they?