Really the distinction here is "where are the airports?". I think you're saying "if you're headed to a place with an airport, you'd fly there directly, otherwise the train makes sense".
Which is not wrong, but also doesn't quite capture all the complexities of flying, especially inter-continentally.
> I think you're saying "if you're headed to a place with an airport, you'd fly there directly, otherwise the train makes sense".
It's not quite as simple as has an airport or not - Nuremburg and Stuttgart do have airports - but major intercontinental airport or not. Airports are and should be set up with their "catchment area" in mind (bearing in mind what neighbouring airports exist), and their transport connections set up to support that.
Frankfurt is the main intercontinental airport for a region with several mid-sized cities, and its transport links are set up to support that. The London airport system is already the busiest in the world, so transport links are - rightly - focused on spreading people through London rather than long-distance services to other places. (E.g. elsewhere in the thread you mentioned Bath - but Canary Wharf, one of the places Heathrow was recently connected to by Crossrail, employs more people than the entire population of Bath).
Which is not wrong, but also doesn't quite capture all the complexities of flying, especially inter-continentally.