What's not sensible, if it gets the trains running, uses the same staff that we have anyway and provides the added benefot of passengers not having to wait in the rain while the guard makes their way down to release the door ?
The only benefit lost is the ability to operate during this un-quantified number of occasions when the rest of the train/infrastructure and crew are available to operate the train but the guard isn't.
And about 10-15 seconds on departure (close local door, buzz buzz, buzz buzz takes about this amount of time). This is insignificant for the Northern Connect services which would be the first to go DOO, but might be significant when you consider something like the CLC stopping services where that could be about 3 minutes on the running time.
I suspect the reason Merseyrail were absolutely not willing to negotiate on guards doing the doors, but were willing to negotiate on OBS or similar, was that this extra time on something as long as a Southport or Chester run was enough to mess up the proposed new diagrams and require an additional unit or two which was unaffordable. After all, simply adding hustle alarms added 2 minutes to Ormskirk-Liverpool running times, as I've posted in this thread before. (It's now had a further 2 minutes added for Maghull North, so now up from 30 to 34).
Edit: actually, correction to that, in the 1985 timetable it was just 28 minutes - that's really quite a slowdown! Of course, back then the doors would be opened while the wheels were still just about turning, and the guard would give 2 on the bell and remain at the open cab door until the train left the platform.