On the flip side of the above post there are also issues which can be created by Doo, equipment could be faulty such as monitors, cameras, RA/CD indicators etc or mirrors could be vandalised. We quite often get e-mails where power cuts have caused loss of power leading to no lighting/ ticket issuing facilities therefore all the DOO equipment would also fail which causes at least a delay if not being unable to stop.
As a percentage I'd think the amount of trains cancelled/delayed by no guard is dwarfed by the amount caused by equipment/signal failures etc.
Also, I don't think its yet been mentioned but what happens to disabled passengers, yes one day level access and automatic ramps may eventually arrive but at the moment that's not possible. Today I've had 2 wheelchairs, 1 to a manned station but unfortunately nobody showed to pick them up so I did it myself, a driver wouldn't have known the wheelchair was there so would probably have gone with them still onboard and the other to an un-manned station, obviously this couldn't happen with no guard on-board.
These things could be remedied yes but it all costs money anyway and that money would take a fair chunk out of the savings of mass redundancies so damaging the purely economic case for change.
Absolutely-I can't think of why a train would be cancelled by equipment faults but certainly DOO defect delays dwarf cancellations due to no guard.