Short answer: Like the others said, it is more on the developers side and/or modders duty because it would be hard to balance.
Long answer:
Historical accurate/realism
Depending on the player historical accuracy and realism is important.
In the german Bundeswehr I was trained on the pistol P8, the rifle G36, the machine gun MG 3 and the anti-tank weapon Panzerfaust 3 (sadly, the last one only in the shooting simulator).
The G36 was of course the standard weapon while the squad mostly got one MG and the Panzerfaust was somewhere in a transport vehicle since it was too heavy for a long march. Switching one of these weapons to a ‘better’ version would be weird, since MG and Panzerfaust are so seldom that you would normally used the better version and replacing the standard rifle with perhaps different ammunition type would make logistics hard. That why we never got a G3 for training and weren’t allowed to field them (differnt caliber).
The P8 on the other hand is classified as ordnance weapon and therefore each soldier can buy its own gun in the Bundeswehr.
I don’t know how that was handled back then in Vietnam War by the U.S. military, but if they were allowed to purchase a own pistol that would be possible.
Gameplay
The gameplay would change depending on the upgrades by progression.
For example, if all players would get pistols at a certain level, the feeling inside enemy tunnels would change. At the last PBS training i realized how practical it is that a machine gunner has a pistol by default while other classes have none. If i have no pistol i would not go into the enemy tunnels since close combat or a bayonett stab are the only hope winning a fight against a enemy inside it. Getting back to historical accuracy thats why the tunnelrats were seen more like the hard-boiled tough guys since nobody wanted to go into deep dark tunnel systems. Giving everybody a pistol would change that feeling, basically making everybody a tunnelrat would feel strange and making certain roles and tunnels less important.
Conclusion
Ok, these were some direct examples what could go wrong in a progression/leveling system.
When we make this system on our own i see the potential failure in either realism or gameplay.
Since we are no game developers, historian or really enthusiastic modders that try to be both, i think we will fail at some point (as the german would say: Verschlimmbessern (combination of the words “schlimm=bad/terrible” and “verbessern=improve”)).
Still, i am open for any discussion about it, sometimes it seems harder than it is. 🙂
The machine-gunners' dreams of point blank fire into serried masses of Emus were soon dissipated. The Emu command had evidently ordered guerrilla tactics, and its unwieldy army soon split up into innumerable small units that made use of the military equipment uneconomic. A crestfallen field force therefore withdrew from the combat area after about a month.
- D.L. Serventy, ornithologist on the Emu War from 11–12.1932 between Royal Australian Artillery and 20,000 emus
Emus outlasted!