Another thing to take into account is the open sections. DLR is mostly elevated or in tunnels, and the rest is well fenced off. London Underground has large open sections where various obstructions, including people, can get on the line. DLR too has train captains who are not only able to operate the doors, but in various circumstances will operate the train from the front in the same way as TOs on LU ATO lines, and they can manually drive the trains in emergencies. It's also much busier, meaning there is more risk from passengers falling infront of trains, as well as problems with boarding.
For fully automatic trains doors would have to be designed to detect if anything was trapped (which could be abused by latecomers to get doors to re-open), and dwell times might be longer, particularly at times of late running (a human operator would be able to see that there was no-one else trying to board or alight and close the doors, wheras an automatic train would be restricted to a set minimum time. At very busy times despatch staff would be needed anyway to close doors and clear trains to depart (I think all but the quietest fully automatic systems have platform staff in control).
As has been said the human element will never be removed, including in the safety critical role, whether it's station staff or those in the control centres, and there would be plenty of those needed for such a big network. Like others have said, these can all go on strike too, as can maintenance staff. We're getting into serious AI before humans can be totally removed from the loop (and by that time it will be the computers that go on strike, or worse).
Emergencies would be another problem. Imagine a train brakes down in a tunnel, perhaps in such a way that it looses communication with the outside world (persuming you would have telemetry and CCTV feed from the train to the control centre and voice and control feed back to the train), maybe due to terrorism. You've then got a load of passengers stuck underground and the nearest competant person is at best at the next station and with no way to communicate with the passengers. No side walkways either, unlike modern systems, so detraining is via the track. With no staff passengers are likely to panic much quicker if stopped in tunnel, even under routine circumstances, unless an explanation was given fast by the control centre. At least with staff presence, passengers will be reassured that everything is under control.
