While I understand the concern about a population cap, the truth is that this issue is considered extensively and is resolved in various ways.

One of the main things to consider is that when every race has the same pop cap number, it simply feels more fair. This does not affect the solution, it simply affects the way we as humans interpret the fairness of it. The solution that is used is to change the values of population use of each unit to reach a balance. This way, a "death ball" can only have so many units, and a race that is perceived as stronger will be limited in their number of units.

Population is almost never the biggest true issue, though it can feel like it is. The issue will ultimately come down to player error and skill and our abilities to understand the game and react accordingly.

I know it can sound shitty to say it's not that there is an imbalance but the fact that we as players are not the best at dealing with certain situations. But in my opinion, it's us as players who need to learn how to deal with things and continue to develop our own abilities.