The basic issue is that a trades union is trying to dictate to a company how that company may run its business.
This is unacceptable - and the RMT is out on a limb. It is attempting to do what no other trades union in the country is attempting.
The other country I know most about is Germany and there the trades unions understand the legal, social and political framework in which they operate - and although they are diligent in protecting and advancing their members' interests they do not try to specify the ways by which the company is run. I know - I was a member of the IG Metall for many years. There is no reason whatsoever - apart from personal intransigence and a distorted view of the world[1] - for the RMT to hold the position it does.
There is a strong moral, economic, social and political need for effective trades unions operating within the law to act as a countervailing force to less than reasonable employers - and there are more of those than one might first think. But this does not mean being pig-headedly intransigent. The political right wing always has a tendency to want to clip the wings of the trades unions, or even ban them depending on the degree of 'rightness'. The danger that Cash does not see is that the continuing strikes will move the boundary of wanting to limit trades unions further towards the centre of the political spectrum. When it becomes mainstream then the position is really worrying.
Effective trades unions are part and parcel of our democratic system - their existence must not be put into question. Democracy does not only exist in politics but is the very warp and weft of our lives.
[1] The language used in its press releases demonstrate this. They show no understanding of the wider world and are aimed only at its own members and supporters. It's an identical attitude to that shown by Donald Trump who's every utterance is designed to appeal to his power base regardless of the effect it has on the rest of the world.
I disagree with the current industrial action, and I don't believe that the RMT would be right to oppose any change to the guards role.
Nevertheless, I am enough of a realist to understand that there is not going to be any major legislation restricting the power of the RMT in the near future, and that the only way to get a working train service is to make mutually beneficial compromise with it.
I also understand that DO/GC has the potential to bring most of the operational benefits of DCO without the strife, thereforeas a pragmatist, I would like to see it brought forward as a way of reaching a quick conclusion to this disastrous strike which is causing so much damage to the economy here.
I am also suitably impressed by @Tomnick's well articulated arguments that DO/GC is at least as good a way of operating trains, if not better than DCO.