Source for that? You need ten buses to replace a single medium-length train. To my understanding trains trade higher up-front costs (infrastructure!) to lower per-passenger-km operating costs.

I mean just based on UK ticket costs I'd say they're not too far off the mark. If you catch a coach rather than the train it's often half the price, but the trade-off is you get stuck in traffic, they're infrequent, if they're oversubscribed there's no standing room (although coach operators rarely face this issue) and they're usually just not as fast as trains. I commuted to work by intercity coach for a couple of weeks and believe me it was no picnic - 5:30am wake time to catch a 6am coach and I often didn't get back until 8pm. Admittedly I was at the end of a route with a number of stops in several small villages nearby which exacerbated things a lot, but still! Some people do this on a long term basis and it blows my mind.

Consumer ticket prices seem like a poor proxy for operating expenses per passenger km. For one, as you said, the quality of service is better.

Even if so, you'd want electric buses to replace the kinds of routes currently served by regular buses - they'd hardly be a replacement for inter-city trains.