With respect, this could apply to anything in life. There is not unlimited money to prevent every eventuality. By your token we should build a separate road for HGVs, because of the potential destruction with a car, pedestrian or cyclist and subsequent loss of life.
If there was a situation where HGVs would take a mile or longer to brake from speeds up 100mph or more, and there was no way of warning them to slow down or stop, then yes, a separate road for HGVs would be needed.
The distances, safety risks (including 750V DC or 25kV AC electric) and energy (trains being 160-180 tonnes for an average 100mph EMU, upwards for longer and faster stock) need different safety precautions.
Let me re-phrase the question. Is it worth spending millions of pounds per year on guards, instead of new signalling, crash-worthy stock, new railways - which also saves lives and creates jobs? Does spending this cash offer good value for money?
Signalling, crash worthy stock and new railways will not mitigate against incidents where the driver is incapacitated or a series of events happen which require two trained members of staff to deal with.
Your suggestions might well stop some incidents from happening, but there will always be accidents on the railway, when they happen, you need trained personnel on site to deal with the incident in the best way possible. The extra eyes, ears and sixth sense guards have will stop incidents too. They know, they can hear and just sense when something is wrong.
ERTMS/ETCS Level 2 signalling won't stop a derailment, ETCS Level 2 provides a little higher level of safety in a handful of scenarios, it matches current safety across the board and in a couple of scenarios, it's a little more dangerous (on risk scoring metrics).
Brand new crash worthy stock can't mitigate against a driver being trapped in the cab of a train, unable to deal with an emergency. You could have a brilliant train with the best cab, but if it derails and ends up in a situation where the driver can't get out, it's still a major problem. The emergency could be unrelated to a crash, it could be the driver being shocked by a live electric wire or rail, it could be an electrical or mechanical failure under the train causing a fire or other problem.
New railways won't stop a derailment, of cows getting onto the line, of a rail breaking, of a cement mixer falling from a bridge or damage to third rail or OLE. There's a million possible scenarios none of which are helped by infrastructure investment.
Guards are an excellent investment in the safety of a train, they go hand in glove with better signalling, safer stock and new infrastructure. They complement each other in making the railway safer.
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And what about the experienced railwaymen that say otherwise?
They're wrong.