I'd like to see Mortars and MGs more useful in the lategame, but i think predicating their usefulness on veterancy might not be the best method. Team weapons are much more vulnerable to being wiped than any other type of infantry, but this is accounted for by their ability to be recrewed. Even with buffs to survivability from veterancy, this still means they'll be just as terrible
Would it be an issue if each faction instead had a similar bonus to Ostheer's BP3 Gren reinforcement reduction? Not in terms of reinforcement reductions, but in terms of an universal "Improved team weapon crew training" bonus in the (later) game, that inherently gave them better RA and/or some other assistance?
Perhaps that's a somewhat "gamey" solution.
The vet is only there to softly account for the vet infantry get as part of their progression. Weapon upgrades would still steam roll support weapons.
Making them cheaper and easier to incorporate in a build through popcap is a way to make them more prevalent in smaller modes.
In case of MGs in particular, the reason since 7 years ago that they lose in effectiveness is that everyone benefits from -50% suppression in the late game. Which is why i suggest to reduce it (i would even go and say remove it at all and for a future COH, create a different type of yellow cover, which is generated by any kind of explosive but doesn't provide any suppression modifier but keeps the acc modifs).