I agree that a frequent (5 minute) bendy bus service will clear most of the problems. I recall using similar services in Europe and it works fast and free. The irritation of having to change mode is relieved by it being fast, reliable and free. As said, the bendys are available and would not have the high service demands of London.
The cost of ANY kind of direct rail link (tunnel/funicular/gondala/spaceship will NEVER pay for itself. They would not be able to charge enough to recoup the investment but a bus service could be running in weeks. In the 30 years I have been using the airport, I have NEVER thought that a fixed rail link would be put in due to cost.
If they were going to do that, the time was in the mid-90s when the new terminal went in. But we all know that the motto of the aiport is 'Do it Cheap and Nasty - Ignore the Future'.