Robin9 wrote:

There was a busy Piccadilly Line service to Hounslow long before the
extension to Heathrow was built. Obviously the Piccadilly will lose most
of its Heathrow custom but Hounslow East and Boston Manor will still
generate considerable business.

Actually, I think it'll keep quite a lot of its Heathrow business, too: for
many people (including me), it will remain the most convenient way of
getting to the airport, even after the full Crossrail service commences.