Glass cannons don´t work well in a game that is focused about obtaining veterancy on your units. Su-76 and Jackson - which I also consider glass cannons - at least have 60 range to dodge shots. Mainline infantry on the other hand should be able to bolster your forces and be able to keep up a fight.
keep in mind that both the rifleman and conscript have terminator vet that significantly boost their durability. Both the Soviet and USF had significant problem with manpower bleed without those bonus.
the grenadier's excellent long range firepower mean that it can more easily punish the enemy while staying out of harm's way. They don't have to dive like the conscript and rifleman do.
stuff like the british mortar and USF mortar broke the balance, but the grenadier themselves are still fine-ish.
Tommies don't get more expensive to buy once you get the 5th guy, and they can pack 2 LMG's per squad. If every gren squad was 5 men, and cost more, then you'd be paying more manpower and popcap per LMG you wanted on the ground.
Brits don't really need to consider this because they need those tommies anyhow for medics and FO teams. You buy grens because you want an LMG squad (and a snare). You buy tommies because tommies are the core of the faction, and then you put LMG's on them because everything is better with LMG mayonnaise.
the british are already paying for a 5 men squad in the beginning. The 4 men tommies are not worth 280 mp.