This could be a very good idea, Let Northolt take short haul / domestic and connect to Heathrow with a high speed rail link/monorail. It could
In the real world, people HATE, HATE, HATE with a PASSION beyond reason, connecting from T5C as it's too far to T5A. Have you read the feedback if busses are used to remote parking? It's utterly beyond the pale! You'd think that they'd had to sacrifice their first born male child.
So suggesting that LHR passengers should connect over the horizon in Northolt via a train, many times longer than the T5 shuttle is not realistic.
Similar tantrums are thrown at LGW if one has to connect to the North Terminal, over there, out that window, in plain sight.