Not having a unit cap would be a very bad idea. The unit caps were not just there to keep performance up, but also to provide balance. It is a well known fact that the power of a unit alone compared to the power of that unit in a group is much different, and in turn the power of that unit in a large group is much different still.
The unit cap meant that you couldn’t just amass one type of unit and use that because it was powerful in groups. You had to create good compositions and utilize each unit type the best you could. It turned it into a combined arms game.
In HW2 the highest unit cap had too loose restrictions compared to the next highest level, which simply encouraged battlecruiser spam over any other composition. The restricted battlecruiser count made them individually more important, made bombers and other anticapitals more potent, and that meant you had to support the BC’s much better. A simple unit cap created so much gameplay depth.