To retro-fit existing non-DOO stock would cost a lot more than it would to keep conductors on the train - and I believe the longer distance services should always have a conductor on board. Anyway as you are a driver shouldn't you be against DOO?
I am against DOO. That doesnt mean I cant talk about how it will be implemented if/when it comes in. I work on DOO lines already and (sadly) I dont think it is a difficult thing to bring about, its just a case of deciding which lines to put it on first. Again, I'm not pro DOO at all. I am aware that the internet can give the impression that you are either massively in favour of something or massively against it when the truth usually is you are neither. I just think that DOO will happen on a lot of services worked by multiple units in the future.
It wont come in overnight, it might be ten years or more before it becomes widespread. I wouldnt worry too much about DOO if I was joining the railway as a guard/conductor this year.
Anyway, this is massively off topic. Back to the OP, I think the easiest way to think about the question is common to lots of works of life. Why does a teacher earn more than a teachers assistant? Why does a police officer earn more than an CSO? A lot of their day to day duties are very similar, but the difference is that one of the people is responsible for a lot more than the other. Its usually the stuff you are responsible and accountable for that differentiates what you get paid and a conductor is responsible for bailing the water out of the boat when things go horribly wrong.