I agree with the general point of the article: that getting in and out of most London airport terminals is becoming hellish enough that trains would be a prefer

I agree with the general point of the article: that getting in and out of most London airport terminals is becoming hellish enough that trains would be a preferable replacement up to 4-5hrs of journey time. It doesn't have to be that way, though. And I'm not sure that would work for Frankfurt, for example. With only hand luggage it is still possible to arrive at Heathrow's T2 for Lufthansa within an hour of take-off time, and when arriving you can be on the tube within 20 mins of landing. The lower-priced Eurowings flights also use the same terminal. If all airports were as well run as T2 the trains wouldn't get a look in.