The efficacy of rifle grenadiers is quite specific to the map, even assuming a fully populated 22 vs 22 server.
I've never found rifle grenade spam particularly problematic:
1. Their blast radius is quite modest and rarely gib their targets. If you're an experienced medic, you can hang back, avoid the blast for the most part, and run in to revive.
2. Engineers with flak jackets can (almost?) take one to the face without dying. You can also switch to rifle grenades and have a duel or something.
I personally have more issues with level 4 soldiers using panzerfaust on 5-10s recharge and carry an SMG on the side. Someone needs to take away their SMG 😀