If you are negotiating an 'Alp worth' of switchbacks on a heavy bike with narrow rims there is a problem of heat. You can cook the brake blocks for them to suddenly have the stopping power of cheese on toast, maybe to send you out wide on a bend (if you are lucky). In rescuing your bike from the vegetation you may touch one of the rims for it to be too hot to handle. Although it is obvious that brakes get hot it is not necessarily an obvious consideration whilst you are otherwise doing a good job of descending.
Broken bones and flesh wounds can be fixed by the doctors and nurses, what you want to avoid is traumatic brain injury where your brain smacks on the inside of the skull, bruises, swells up and leads to brain damage. With age brain damage becomes less sustainable in that you cannot recover from it in the way that those that suffer strokes recover brain function. Allegedly just falling over onto a hard surface can give your head a 15 mph whack, enough to ruin your brain for maybe a lifetime. A helmet will not change this sudden deceleration force significantly - your brain behaves much like a person not wearing a seatbelt in a car where crumple zones in the bonnet don't really help. Technically standing still is therefore 'dangerous' - you could faint and die, just like that.
'Feeling safe' has nothing to do with this technicality, but hey, if you are in the zone, on a cool road with no mechanical/traffic/weather/surface problems then 65 mph 'feels' fantastic and if you make it to the bottom in one piece then '65' is obviously safe...
Perhaps a more logical way for a UK cyclist to look at the situation is the 'twenty is plenty' road safety campaign. This campaign is all about pedestrians and setting traffic speeds that are survivable for them. Getting hit by a car at 20mph as a pedestrian is probably similar to falling off a bike at 20 mph - a lot more survivable than 30 mph.