Even if you think what you would say is obvious, please add. This is genuinely something I think makes sense regarding local bus routes given the longevity of light rail and how infrequently routes change, but I also suffer from confirmation bias, so I’m hoping for reasons this would be a terrible idea but obviously would prefer reasons it would be an even more amazing idea than I thought.
Well, like I said, I agree buses aren’t a good replacement, but they’re comparable in the way they’re used in cities: as a short to medium ranged local transport. You wouldn’t want to take the bus from one end of Berlin to the other, you would take the S-Bahn, because that’s what it’s for.
Compare the bus network with the tram network in East Berlin. The buses (usually) run where the trams don’t, but they have a similar amount of stops. Of course there’s overlap between the modes, but the general idea is: tram/bus for short to medium distance, S-Bahn for medium to long distance, and U-Bahn bridging between them.
They’re also comparable in accessibility: with the U- and S-Bahn I have to enter a station. With the tram, I just step out my house, go to the next corner, there it is. Same with buses.