That's the point of it- you can't rely on AGrens only once the Soviet player starts transitioning out of Conscripts, or you'll be in trouble mid-late game- or you could exploit the synergy of the doctrine with T1 skipping and go 3X AGrens -> Scout Cars to counter the sniper.
What's the point of making 3 units that will be literally useless once early game is over because of the implied cost when they enter combat? They're CQB units, so they will end up losing squad members and that will hit your economy hard. It's not like they're as effective as Shock Troops so why would they cost the same to reinforce?
I'm assuming the three AGrens won't win you the game alone (and if they do, that's not balanced whatsoever), in which case they will simply be a huge liability as the game goes on. I'd rather just get regular grenadiers and go for PGrens as a shock unit - at least they can scale into the late game as AT.