You're not arguing regarding Penal's combat effectiveness at this point, but merely their affordability.
Affordability of unit directly affects its combat effectiveness, because in infantry case, field presence of a unit is quite important, cost and timing influence that effectiveness.
This is why penals are alright unit, but dead as meta one, while PFs are being spammed everywhere above 1v1.
Since you don't start with premade army matching opponents premade army for the pop and cost, but need to field it first, you have to take into account timing, cost and what opponent will have on field as well.
Penals will ALWAYS be outnumbered, they will win 1v1 engagements, but they aren't going to be engaging like that past first 2-3 minutes, because its impossible to field enough of them.
And, again, we've made a full circle, where T-70 comes to the rescue, balancing that out in mid game.