They also have 40 range to the Shreks 35
Edit: Also AT rifles don't greatly reduce the squad's DPS. It keeps it about the same.
The entire reason you get Grens is for the LMG42. Nobody will shell out munis to lower their core AI squads anti infantry ability for light vehicle countering in the faction with the best light vehicle counters. It's ridiculous proposing Ostheer and OKW be able to choose(note that they have ubiquitous access via good teching) between mines, Shreks, AT guns, incendiary rounds, fausts, 222, your proposed AT rifle upgrade, and Puma. This is not something that the Axis needs, and outside of some weird Volks spam builds with call-in infantry thrown in, it won't be used because there are tons of better vehicle counters available at the same time that don't have as high of an opportunity cost.
If you want more upgrades for core Axis troops, MP40/Faust upgrade for Volks could work (as in, maybe) if you nerfed JLI and Pfusilier. Grens already have additional upgrades because of Ambush Camo and G43s. I'd be down for a G43 buff if you increased the cost to like 100 munis.