Regardless, the difference between a Guard’s competency and an OBS’s competency is greater than just “doors”.
(and, as lighthearted as you make it sound, that’s not exactly trivial when you consider the various reports of passengers being dragged to serious injury/death)
However, a train that has been rostered to carry a guard, conductor or OBS who for any reason isn't available currently won't run? If it's the last train of the day and that member of crew is missing the train may have to run ECS to wherever it needs to be at the end of the day?
Seems silly to operate an empty train, leaving perhaps only 5 or 6 passengers at various points along the route? OK lets let them travel, this once. So the thin edge is inserted. Within 12 months that service would be operated driver only 2 or 3 times a week - and before long it's accepted as full DOO. Probably no accidents, if all goes well. Probably?
I'm afraid there aren't easy half measures because most of us with any length of work experience know what happens next. If you're in management you want to push it as far as you can. That lightly loaded last train may be OK once in a while. The rammed full train at 17.30 may not be. Yet it's been done elsewhere for decades!
Now bringing in a specific example, the last train from Manchester to Sheffield due to stop at Hathersage and Grindleford. It's usually lightly loaded. It's often late due to a variety of problems built up during the day. It runs through a lot of cuttings in open country and two long tunnels. It's running with only a driver, stopping all stations. No, it doesn't hit an alien or Dr Who but a fallen tree in the same place and the driver is just as incapacitated. There is no mobile signal in the cutting and on the embankments in and around the woods between Hathersage and Grindleford. Passengers are on their own to find their way along the tracks to one of the unmanned stations. Some may know which way is best. The woods are a bad option, even in daylight for those who've ever walked or worked in there! (It's where Network Rail was going to build a loop for the Hope Valley Capacity Scheme until they found how difficult it would be for both construction and access.)
It's thousands of special situations such as this across the north that are at the heart of this dispute. The RMT waves the shroud of a possible Grenfell scale disaster. In 50 years we all hope it won't happen, but it's the nature of accidents that they do happen, somewhere, and with unpredictable results. Even when one does happen it's by no means certain that a fully trained guard would cope better than an OBS - or even members of the public left to use their initiative! But clearly they should.
Management of risk is not, and never can be, a totally precise science! Accounting for the most obvious and the most unlikely risks will never be totally agreed, so this dispute may indeed go on for ever. Encouraging thought, isn't it? We'll just have to use our cars - vehicles that are a lot less safe than taking a train.