Unlimited pop would help the player that gets ahead and starts to roll out mass vehicles first, since they inflict mp-damage without receiving any. vehicle armies will grow much more linearly (only affected by upkeep) as opposed to balanced or inf armies naturally stopping growth (due to upkeep + reinforce).
Currently you can see that effect to a minor degree - armies with mass mediums start to float a lot more mp, against armies with balanced comps- which take mp bleed in exchange for map control. The 100 pop limit helps stop the vehicular army from turning that MP saving into even more units, which inflict more bleed more efficiently, snowballing over the nonvehicle army
I'm skeptical of whether this makes for good gameplay. with infinite pop maybe players might start suiciding low val squads or leaving them unreinforced to capture after some time, to just maximize a MP-efficient vehicle army.
You're forgetting that engineers are additional upkeep to vehicles making their true pop much higher.
Higher value vehicles benefit from this mechanic once again since they have better armor values, and bounced shots don't need to be repaired.