It would seem to me that the single most important thing in a sidearm other than reliability in the military is longevity. How many years can it reliably work for a decent price point while still being a decent weapon.
Look how long the military stuck with the 1911. And it still almost is good enough in that role. The more of the next century the next handgun can last the better.
Face it a sidearm is something a small percent will ever rely on, and these guns will be shot a moderate amount to use up ammo each year and worn or moved about in abusive fashion a lot. They need to be light enough to not be a pain to wear, reliable enough to go bang reliably in harsh conditions, but last a long time so nobody has to keep buying handguns of little importance to the military to replace the worn out ones.
Handguns require precise shot placement to be useful in a military role where most people have rifles but are often the most neglected pieces of gear the soldier will have because they are not priority.
I don't like the Glock as much for the military.
Even though it is really cheap for big contracts, reliable, and probably the least trouble to carry it relies on Glock the company to replace damaged or overly worn plastic frames, because quite frankly a plastic frame may work for a huge round count, but can not take a lot of direct damage. What turn into dings or scratches on metal and remain cosmetic are gouges and structural issues on plastic. It is also more sensitive to temperatures and chemicals.
Glocks can even be reshaped in boiling water. That is just not military suitable in my opinion, your sidearm shouldn't melt if you have it next to a hot motor, machinegun barrel, or crack or break because you land on top of it while carrying a full kit. Yes I know plenty of military units use them, as do many police. I still think for our own military should have something that is more robust.
The more modular the firearm the easier it should be to swap parts. So the military can keep some running a lot longer with spare parts and parts from cannibalized ones. That is cheaper to the nation to provide a high quality firearm to its troops. You want to be able to stick with a handgun until some technological leap requires them to need an update. Right now a handgun is a non body armor piercing sidearm that is not particularly effective per shot on people that will typically be armed with long guns, often body armor, and close to you if you are within effective handgun range and willing to engage them with a handgun.
So using a handgun already sucks in combat and is mainly useful to MPs and special units.
We have a big military that needs handguns that will last a long time, can take some abuse and make reliable sidearms.
That said I love a Glock for civilian use. They get less abused carried by private owners, and are reliable and light weight and can go a huge number of rounds while at the same time making good shooters. Good weapons that can be carried easily.
Privately owned Glock the company at this point in time with its very old owner is a company that honors its product.
But things change, Gaston won't be around much longer, things may be changing for Glock, and the military is really better served by a gun you can drop on the ground, stomp on, drop a 80 pound pack on and then pick up and use, after previous soldiers issued the gun did something comparable during their service. Handguns need to be tanks in the military, and I dare say the aluminum frame M9 was barely enough and are getting to be rattle traps at this point. Yet they are certainly tougher than many popular civilian firearms. Something steel even tougher is probably a better direction than something plastic. Look how robust the m9 is for a 9x19 on the civilian market yet is still taking a beating in the military.
Now if I was that individual soldier I would rather carry the light weight plastic (and have it properly maintained and swapped out when it wore out), but logistics is very important in the military and I think the steel guns if treated appropriately will outlast them. Now when you get to rifles the size is significant enough that weight saving material starts to make more sense than hamstringing your troops with a rifle that will last a century but weighs so much it reduces their mobility and ultimately how much they can get done. But with a handgun the weight savings I don't think make up for the trade off in its ability to take a beating for general use.