There is no handgun in the world (regardless of caliber) that represents a significant enough improvement over the M9 to justify its replacement.
Really?
People often say that because it is in fashion to say so, but actually they only say that because they're not the one being stuck with it.
It's a feigned wisdom with no real substance. What constitutes "significant enough" improvement anyway? While in the military, I shot my M4/M16 better with iron sights than most infantry troops shot with Aimpoints. So, are you going to argue that those Aimpoints that can cost nearly more than half as much as the gun should not be bought because the improvement is not "significant enough" too?
I can tell you right now that if there were two of me, and they fought against each other in completely equal condition with one holding M9 and the other holding my pistol of choice, the winner being the one holding the M9 would be very improbable. I don't know what constitutes "significant" for you, but that is significant for me.
Why don't you walk upto NAVY SEALS who took down Bin Laden and tell them how their HK416 is unjustifiable since its improvement over M4 is insignificant too? Can you explain that?
There are number of major manufacturers: SIG, H&K, S&W, Glock, Beretta.
Out of dozens of pistols pumped out by them, Beretta come out as bottom of the heap when it comes to agency choices outside of the army.
As far as cost and priority goes, as long is it is off the shelf, I cannot see any significant cost increase associated with it.
Why? Because the military ALWAYS needs to buy some pistols. Even if no new model is adopted, military is always buying replacements and parts. And, from what I've seen, M9 requires significantly more maintenance than pistols like Glock 17. I would not be surprised if M9 would exhaust a whole lot of replacement parts or broken parts before a Glock 22 would need any part replaced. I am not a fan of Glock, it is just a frame of reference.