One problem is that this plan is bad for everyone in the half of metro closer to airport or coming from other direction. They would have to go out of their way.

One problem is that this plan is bad for everyone in the half of metro closer to airport or coming from other direction. They would have to go out of their way. Or the airport would need checkin facilities, then what is the point of the downtown check in. The other problem is that regular trains are better because they stop at intermediate stations and can run more frequently because serving regular traffic. The Elizabeth Line is better than the Heathrow Express. The only way I can see it being advantage is the check in and security is done on the train. That would save time, But that would require a train twice the size with pre and post security areas, and luggage space.