On the plus side, I'm glad to hear London has active sewerage surveillance in place
Some US cities in the middle are like this, nobody (to a first approximation) lived there in 1901, and today there may be a small city with an airport and a decent night life. But in Europe, and on the US coasts, the cities are older.
In theory, you can separate existing combined sewers. For a town you could maybe attempt this as a large local infrastructure project over the course of a year or two, in a small city this is a large and expensive project that could take decades to complete. For somewhere like London or San Francisco it's unthinkably costly and won't be attempted in the foreseeable future.
A small sign of willingness to attempt this, combined with the difficult reality of such work goes like this, I live in Southampton, an important port city on England's south coast:
1. Legally, if you fit a new grey water waste (e.g. from a shower, or a washing machine), it should flow into the sanitary sewer, which your home is plumbed into usually via large black pipes that run mostly vertically up the side of the building. It must not flow into the storm water sewer, which is often smaller rectangular pipes connecting to the gutter where they collect roof water.
2. But, there's probably a gutter pipe right near where your waste is, while the sanitary sewer could be on the far side of the property. So, does the plumber do the extra work, and charge you for it? Or do they just cut into the storm water sewer and save their time and your money?
3. Result: Now if you separate the storm water sewer, it has grey water waste flowing into it, and you're dumping that into the sea. Oops.
The big problem in San Francisco is that during severe rainstorms the flow rate overwhelms the treatment plant so they have to dump the overflow. but at that point it is mostly rainwater so it's not quite as bad as it sounds.
For example, how many cubic metres of rain water makes adding one human turd OK ? A million seems fine, doesn't it? Like, who cares about just one turd in so much rainwater. How about a thousand, that's a swimming pool (not Olympic, but decent sized) with a turd in it, is that OK? Start to feel a bit uncomfortable with how much shit there is in the water? What if it's a hundred? Are you sure San Francisco, or London, can promise you their CSOs have at least that ratio?
(Disclaimer: I vaguely worked on software projects that were CSO mitigation adjacent. Which at least explains some the interest in the projects.)
Then in the event of heavy rainfall there is much less poop in the overwhelmed (mostly) rainwater treatment plant.
# Solid human waste will clog the smaller pipework and now your toilet waste is overflowing inside the home and the occupants are very angry
# The fittings don't "fit". The local plumbing supply store has the adaptor you need to run grey waste into rain water drains, even if it says that's not what it's for, but it does not have adaptors to run a large diameter soil pipe into the storm drain, that's a recipe for disaster they want no part in and the part has no other purpose.
If you indicate that you've got some "good" reason to attempt this, they'll sell you an expensive macerator which solves the first problem by grinding up waste, and as a result the output is smaller diameter waste pipes - but now your project is more expensive and more complicated to install.
One segment at a time, I guess. But like you said, it gets really complicated in older cities with many hundreds of years of things having been put in the ground. Anyway they can probably process that just fine, it just means that at rainy days there will be higher water content; it means they'll have to scale up. Or if there's overflow, dump it directly if they're already doing it now; if they can reduce the amount of sewage going into surface water, it's already an improvement.