In the case of both Northern and Merseyrail any DOO/DCO would come in alongside brand new trains being introduced, so it wouldn't be a case of no guards and nothing in return for the passengers.
Is passenger safety and accessibility are the number one concerns then a more constructive approach would be to work with TfN, BTP, local police forces, RUGs and disability groups to come up with a list of concerns. For instance, is there evidence that crime is higher on DOO trains, are vulnerable passengers more at risk at unstaffed stations or on DOO trains, if there isn't a guarantee of a second member of staff on all services would disabled people be able to use the train etc. It might not come up with the result the RMT want on all lines but it would then be strong evidence unlike the RMT bleating "DOO bad, guard good - we want an independent report done (not at our expense) to prove it as we don't believe the RSSB report which states DOO is safe if properly implemented."
The biggest problem with the RMT's DOO disputes is they've started a dispute with every new franchise - in some cases they've tried to start dispute with the old franchise over what's in the ITT for the next franchise. They've even tried to to start a dispute with franchises which had no plans to get rid of or change the guard role 'just in case.' Having the rep go to the new franchise holder to seek assurances is completely reasonable and expected but reporting it as a dispute with an evil private/foreign owned company (delete as applicable) and immediately balloting for a strike if a 100% guarantee for the duration of the franchise isn't given, is not using strike action as a last resort (the way it's suppose to be) and just proves Mick Cash has a big ego. I've been saying for a long time that if the RMT don't grow up the government will look at changing legislation which will make other trade union members suffer for Cash's arrogance, fortunately it sounds like this idea will only class public transport as an essential service opposed to making it harder for anyone to strike.
Thing is it's the government's job through the DFT to look at how DCO affects safety and the safety and accessibility for customers. Presumably they have done that and have decided they wish to push on, as they are writing it into franchises.
Disabled collectives themselves have put their concerns over to tocs over DCO by themselves bit it hasn't guaranteed the staff in many cases.
But that is all directly DCO related and doesn't have any bearing or the staff's right to strike in the UK, and the reasons which include reasons of wanting to protect their jobs, their livelihoods, the likelyness their job will actually be required in the future, whether or not they will even be required as a guarantee as part of the operation of the railway or an optional extra, whether they will lose their safety accreditation and become ticket clerks, whether there is reasonable need to change established working practices and methods which have been established since the railway began and which the railway has run fine with until now. Add to that the issues you mentioned about accessibility in terms of staff may not be guaranteed in future as they are currently, and potential vulnerability for many customers travelling alone when the only staff could be the driver. The DFT knows going against all that is going to cause mass grief for staff and the public. They have weighed up the for and against and seem to have decided on some or many occasions it's worth pressing on.
The point was made that in exchange for the DCO dispute the public will get new trains. True. But do the public accept that as compensation for suffering over the dispute? Or would they rather the TOC just put an end to it by keeping things as they are, introducing the planned new trains but keeping the established method of operation as it is today? We all knows guards panels can be fitted to all the new stock if ordered.
One final add, it makes the whole thing look more daft in England because the Welsh government are guaranteeing the guard and supporting it, Scotland also support the second member as being essential, the intercity tocs have widely decided to keep the guard now so it's just some tocs and not others. It fits some but not others which just seems unfair to those who live in the areas where the DFT have decided this operation should exist.
If there is a viable reason why the current method of operation with a guard is not suitable in the future then why arent the DFT proposing it's withdrawal as standard for all Tocs and all countries in the UK? They're having a job explaining why it's ok in some areas like the North but not in the South West or Wales.