>>2040819
It's finding the sweet spot that fits for a particular city, now and into the future.

Excessive standardisation leads to transit modes being ok at what they do, but not doing especially well. Also, in S-bahn systems, delays can spill-over very easily and Thameslink is a prime example of this. A 5-minute delay in Peterborough can result in delays happening as far away as Brighton.

With medium-sized cities, your idea might work very well. But a large city will need all the capacity it can get and only a dedicated metro system can do that and avoid delays from spilling out onto the wider network.