Well, a lot of the time, London is exactly where the business traveller from Essex wants to be
I should add - It is utter madness that Crossrail is not continuing at least as fas as Reading, which is a major junction and a key commuter destination (in and out).
Electrification to Reading would also enable a Reading to LHR through train, instead of the horrible bus link provided at present. Given that Exeter St Davids to Reading takes about 90 minutes, I think this train connection would be much more convenient than flying, especially as there would be no room for BE in T5, even if they wanted it.