I don't have to imagine, with practical experience with DOO across quite a wide area of the network. and no vested interests as I've now left.
Why it's safe:
It already happens: GWT, C2C, Chiltern, Southern, Scotrail and so on. The Underground has a form of it. Billions of people have been carried since it was first used in the early 80s, we have actual data. This is why RSBB and the ORR can conclude it's safe and prove it. It happens in other countries too.
We have one of the safest railways in Europe
The chances of anything happening is remote on passenger/train interface, very remote, not far off one in a BILLION. Even so, that has to be offset against risks splitting dispatch: Reaction times and so-called ding ding and away and you still get trap and drag. Anti-DOO people neatly side-step the death involving guard operation in Merseyside.
GSMR/CSR is still better than the alternatives: SPTs, flags and dets. TCB (a prerequisite) is better than Absolute block.
My experience tells me so: I've worked at many locations using it across a wide area of this country, never was there any concern again, nor were there mishaps. In fact, I stopped two major mishaps using the system because the trains involved had radio.
Money is better put into other things that pose more of a hazard on the railway: Level crossings are one example, securing earthworks is another. Off the railway, there are countless better ways to save lives.