Question: When CSOs were first built, was overflow not a problem back then? Also, I’m wondering if there are people who are “pro-CSO” because they also lead to stormwater being treated?

Good question. Most systems that are now CSOs developed piecemeal: Before wastewater treatment requirements, sewers (whether combined or separate) simply discharged to the nearest water body without treatment. Then when wastewater treatment plants were built, interceptor sewers and pumping stations were built to take all the water from the local sewers to the WWTP (wastewater treatment plant). But that infrastructure had limited capacity, so for combined systems, the relief valve in large events was the outfall where the sewer had previously discharged all its waste, i.e., CSOs.

In terms of your second question, yes, there is some value to treating stormwater to reduce water-quality impacts. So combined systems with adequate capacity are viewed by some as superior to separate systems. However, IMO shunting all the runoff from the urban environment into a WWTP and then discharging it at one downstream point is usually inferior to having that runoff move slowly into groundwater and local waterways through green infrastructure.