So stop spamming them and use snipers or HMG42 or 251 or PGs as the faction is intended to do.
You want to spam basic infantry and ride on that, you have OKW and USF.
Well, let me see here genius. What else should I build? Oh yes, 4 men pio squads, 4 men pgren squads, 4 men everything squads. Grens don't carry over to the late game at all, they underperform there, but they are supposed to be the backbone of your infantry core. Furthermore, they are expensive to reinforce and being only a 4 men squad don't have any staying power, like 4 men tommies have who are far tougher. Those were considered to be "slightly overperforming". If we use that insanity, we can fairly savely state that Grens are "slightly underperforming". Soon you battle 7 men cons as well at an earlier timing too.
Sniper. Well yes. Soon you will have to face 7 men squads with that too. Not to mention that all snipers are now just as fast. Meanwhile, your 4 men infantry squads have a hard time against anything. Face it buddy, grens are lackluster still and you sweettalking around it doesn't work.