Is it better to fix the problem now or spend decades doing it properly? Consider how much money it would take to dig up every street, add new sewer pipes, and coordinate disconnecting the rainwater pipes.
In Portland, we did similar combined sewer pipe. But they didn't size it to handle the largest rainfall. The sewers still overflow a couple of times a year, but it is much better than dozens of times. Especially since it now only happens in the winter when no one is using the river. It would have cost a lot more to handle peak rainfall. It is already expensive, sewer costs twice as much as water here.
In Portland, we did similar combined sewer pipe. But they didn't size it to handle the largest rainfall. The sewers still overflow a couple of times a year, but it is much better than dozens of times. Especially since it now only happens in the winter when no one is using the river. It would have cost a lot more to handle peak rainfall. It is already expensive, sewer costs twice as much as water here.