There will always be a requirement for short haul aircraft to feed the long haul network, particularly at the largest hubs, simply because it cannot be possible to fill all the seats on the long haul networks purely with transfer traffic from other long haul flights, or from the locality around the hub itself.
In the longer term, presuming that the improvements are made to the railway network westwards and northwards, then LHR may become slightly less reliant on the short haul feeder network than it currently is.
However, that is a big IF and and even bigger WHEN...!