Rather agree about internal flights. However, even now we have trains managing London to Manchester in only a few minutes over two hours.
Even being quite optimistic, getting checked in, boarded, flight, through the terminal at the far end can easily take that long. Quite often, getting to/from the airports further adds substantially to the effective journey times.
My last Scottish train journey included Newcastle-upon-Tyne to Edinburgh which at that time seemed to manage about 50 mph. (It looks now to be more like 80/90.) So even 125 would make quite a difference.
300 mph (and similar) always makes me suspect that the number of stations served will be very small. It is bad enough on 125 trains, but at each increment upwards, there is greater reason to avoid stops.