It will be interesting to see what the drivers say when they're asked to work trains DOO that previously carried guards - personally I think that's going to be the deciding factor.
Where it is already in the contract, relatively little, unless there are local agreements not to extend it.
Where drivers do not have it in their contract (i.e. intercity operators and most regional stuff outside the South East), outright refusal and probably industrial action. No one down my part wants anything to do with DOO, they don't want to be jailed when drunks and late runners slip, nor deal with anti social behaviour, criticism and moaning, and violence, whilst attempting to concentrate on a job which one screw up means at best being on the first steps to dismissal via plans, or at worst death.
Many transferred to get away from DOO and have many stories of near misses on the platform, transporting around gangs, drunks, criminals and being attacked, spat at and abused. Many were ex-guards previously. I was talking to a DOO driver who admitted he almost missed a station because he was thinking about the threats of violence he had received off drunks, rather than concentrating on driving.
We are told that distractions to drivers are a major issue - no one allowed in the cab, do not allow yourself to be distracted by pouring tea, passing trains on other lines, signals on other lines, people waving on bridges or whatever, yet drivers are expected to deal with passengers - of which 99.99% are reasonable decent people who deserve a good service, but of which 0.01% are a nuisance / distraction / selfish / dangerous to self and others, and of that group a percentage are basically criminals. How it is unsafe to have another driver or guard in the cab, or talking to the station dispatcher because it results in distraction from looking at signals, yet dealing with anti social behaviour doesn't, I have no idea!
Interestingly a cleaner once told me that DOO trains were always filthier and in a worst state and full of far more unpleasant things than trains with guards. Presumably some of this is reactive - a guard may do a bit of litter picking or report things before they get out of hand such as blocked or fouled toilets, and biohazards, which a driver would not know about, nor have time to report, but it appears that on trains where there is no staff presence, nor threat of removal from the train, it is as we have always said, a free for all for the very worst in human behaviour - and the poor fare paying public have to be surrounded by it.
Most drivers are on decent money now and certainly where I am, care more about conditions. DOO, compulsory Sundays (unless it is a 4 day week - rather than the 7 day week that the TOCs want) and 12 hour movement off spares are always things that will be thrown out of any pay talks, no matter what is offered. It's already been tried and been turned down multiple times.
Finally there may be cases where drivers are permanently removed from driving, either for medical or due to being unsuitable. At least they may have the protection of going as a guard or platform dispatcher, rather than the dole queue.