The difference between a supersonic jet and and a subsonic jet is about 2x. Between NY and London that's a 3.5 hour flight vs a 7 hour flight (plus the overhead of security, traveling to and from the airport, boarding, taxiing, etc. which brings down the proportional cost).
By comparison, a boat would take 7-8 days. The disparity in time saved between supersonic and subsonic flight is pretty trivial in comparison to the time saved between a boat and a subsonic plane.
Presumably there are no flights from Paris to Munich then?
There are. A flight is about an hour and a half, a train is about 5 hours. Far more people fly between the two every day than take a train.
A 5x disparity is still a lot more than a 2x disparity. In fact, considerably less than 2x given the overhead time of traveling to the airport, security, boarding, etc.
How many people would pay 8x the ticket price for a 45 minute flight from Paris to Munich (The Concorde was ~8x the ticket price of economy subsonic flight tickets)?
If that were true, we’d all be taking trains and boats everywhere. We aren’t.