I think we should stop being fixated on payment type and focus far more on "usage when".
The fixed DD in theory is balancing from what I was told on an annual point where your meters could be read so if properly taken and no massive price changes the net impact of the fixed DD customers should be basically zero.
I do agree however in that its quite likely that it will become a privaledge rather than a right as some point, eg being credit scored for an account.
What you miss is that there are significant costs incurred with prepay so its not the cheapest to support at all. In fact its considerably more expensive.
Maybe if they paid the actual fee for the payment then it may support a cheaper price, but with potentially 20-30 transactions per month compared to the other two methods its completely obvious why its more expensive.
The middle option has just as much risk as the last unless its based on a smart meter.
In fact a smart meter should get a discount due to accuracy of data provided.