It's a pretty complex stat in practice, I wouldn't be surprised if any thought of it was quickly responded with "How about no". The Manpower resource affects literally every unit in the game. A manpower income disparity is much like the higher side getting a (manpower) free infantry unit at a given point in time regardless of the enemy's efforts. We can all look back and think of times when a lost unit probably sealed a match's results.
This advantage would also persist to some extent to later in the game, when a full population makes reinforcing your individual infantrymen manpower harder and probably turns manpower into the primary resource for replacing tanks.
...though doing this in some way as an upgrade is definitely a good bit more feasible. This was essentially what CoH1's Americans' Supply Yard's upgrades reducing upkeep did.
Yeah but you also had Zombie Armies there, especially the Wehr, they could turn dead Volks and Pioneers into Grenadiers even without the Tier 2 building.