DOO isn't 'safe' in exactly the same way as conventional operation isn't 'safe' - nothing, ever, can hope to be absolutely free of risk. There's no doubt that there's considerably more risk attached to DOO operations - most of it has already been touched upon on this thread - so the question for the accountants is whether the cost saving justifies the increased risk. I remain far from convinced that there will be an overall cost saving, from a subsidy point of view at least, anyway: the reduction of staffing costs won't be significant if each train carries a revenue/customer service bod anyway (yes, they'll be paid less, but how much so?). If the franchise is let on that basis, there'll not be much saving to be had (and if not, why not - do we really want to see regional trains running between, say, Liverpool and Newcastle with no-one but the driver on board for the whole duration?) - though no doubt there'll be provision to allow a train to run without the additional member of staff if necessary - providing a real opportunity for the TOCs to direct improve their bottom line by deliberately running staff numbers down and leaving turns uncovered rather than providing spare cover.
On a wider note, I can't see the general theme of destaffing and pushing for wholesale cuts to pay, terms and conditions ending well. What happens to those left unemployed by the former (joining the ranks of, for example, the supermarket cashiers made redundant by the self-service tills that are continually promoted), especially as railwaymen have a fairly unique skillset that's not easily transferable elsewhere? Those who keep their jobs, but are forced onto a much lower rate of pay - suddenly they have much less disposable income, so their spending decreases, to the detriment of the economy as a whole.