There are certainly some diminishing returns with high speed air travel. If it takes you, say, at least an hour to travel to/from the airport in the best case (e.g. no security wait or using TSA precheck) then the difference between a Mach 0.8 and Mach 2 flight isn't that huge percentage wise, especially when you factor in the time it takes to get to cruising altitude. On extremely long flights it would make a bigger difference which might result in a more comfortable flight but it doesn't fundamentally change whether or not the flight takes up most of an entire day or not.
That doesn't change until you get into extremely fast travel such as ballistic point-to-point transport. When you have flight travel times that are on the order of an hour or two (including overhead like vehicle loading and unloading) then it becomes possible to integrate them into a day where they are just a part, which opens up a lot more possibilities and might justify the expense.
That doesn't change until you get into extremely fast travel such as ballistic point-to-point transport. When you have flight travel times that are on the order of an hour or two (including overhead like vehicle loading and unloading) then it becomes possible to integrate them into a day where they are just a part, which opens up a lot more possibilities and might justify the expense.