It's an interesting proposal, but we don't buy huge quantities of weapons for a single environment such as urban warfare. Right now, that is where most of the combat takes place. It will be that way in any country where you first invade and then hang around. Initial combat takes place in the more open areas where armies can maneuver and fight. But part of being the winning side is you achieve air superiority and the ability to overwhelm the enemy in the open. Consequently, enemy forces will scatter in urban areas and resort to small unit operations so that you can't use the overwhelming might that you initially used to destroy the army.
That said, I think the AR/M16/M4 system offers greater versatility than any other weapon system out there. Here is my reasoning for my choice.
First, the primary configuration should be a 16" midlength in 6.8x43 with the LW piston system and a collapsing stock. I like this combo for a number of reasons. 16" offers a reasonable compromise between the longer range capability (increased velocity) of the 20" and the handiness of the M4. I don't think sight radius is as important as it used to be with the widespread issue of optics. Midlength gives increased handguard real estate over the M4 carbine length gas system and the gas system improvements. The LW piston system also improves control and cleanliness. 6.8mm offers increased lethality over 5.56 at all ranges, especially the longer range. It bridges the gap to 7.62mm. Collapsible stock allows for adjusting the length of pull for body armor, etc.
I think the AR system is more versatile than the alternates for a number of reasons. First of all, it is more ambidextrous than most. Most rifles have a charging handle on one side or the other. If it is on the left, it is very handy for a right handed shooter, but less so for a lefty. It also has the potential to snag on gear, 3 point slings, etc. If it is on the right side like the AK, SIG, etc., a right handed shooter has to reach over or under to reach it. The AR charging handle is equally accessible to left or right handers. The bolt release and mag release are not ambidextrous, but those are not as important an issue and the only ambi mag releases are paddle types, which aren't as fast as the trigger finger button.
I'm not a real big fan of easily changeable barrels for one reason. Zero. If the only thing you change is the barrel, the new barrel will not be zeroed to your sights. If you change out the whole upper receiver, it can be zeroed and then set aside. With this in mind, I'd like to see units get a number of extra upper receivers issued for more specialized use. Some 10.5 or 11.5" uppers for CQB ops. Maybe a 20" upper for a designated marksman.