pMk1 stock only has a 7%g braking rate, in common with other tread-braked vehicles such as Mk2s and 150s, whereas disc braked stock brakes at 9%g and most do 12%g in the emergency setting. This could occasionally make the difference between a near-miss and an accident, but other than this I don't see any reason why Mk1 stock makes the railway any more dangerous or indeed more likely to be disrupted from the point of view of somebody not on board that train. With less to go wrong a train of Mk1 stock probably causes less disruption than newer types, though the traction may be a different story. Passengers in Mk1 stock also benefit from TPWS and many other safety improvements that have come in since it was introduced.
Comparisons with driving are relevant because the average car journey is around five times more dangerous than doing the same journey by train, and has a much higher chance of affecting innocent bystanders. Any difference between travelling in a Mk1 or a newer train would be tiny by comparison. Motorbikes are more dangerous by a factor of several, and of course a smoker has about a 30% chance of dying from it... but most people who partake of these activities are aware of the risks and choose to take them.