Since they cost less and they face elite-infantry, they are not suppose to fight them, like I hope grens are not meant to face shock troops.
The tournament also showed that Ostheer rely heavily on Ostruppen doc, or Agren, or Infantry doc, but never on vanilla gren.
In 1vs1, it seems you want to avoid grens (for ostheer) and mortar (for everyone).
By your logic I guess british Assault officer is underpowered too? He was not built once despite tommies seemingly to struggle vs those Ostheer upgrades or callin inf. (win rates support that with brit win/loss vs ost).
Countary to that we actually did see grens (yes upgraded as you said), and Ostheer won a lot of games. I'm not seeing where the issue is here in balance.