Cars equipped with Auto-Hold do not require one's foot to be maintained on the brake pedal. The car will remain where it is with feet off pedals until the fuel runs out unless the EPB is engaged.
I should imagine that if a driver becomes incapacitated while the vehicle is at rest, the potential for disaster is considerably less than at 60 or 70 mph on a busy road.......if such incapacity isn't actually calamitous in itself by definition?
Mine does not have auto hold nor EPB so if in D with no handbrake the only thing that is stopping the vehicle is the foot on the brake. If these fearutes are equipped, it is anyway foolish to rely upon such contrivances since they can fail.
WRT incapacitation while stationary, I was thinking particularly of the usually temporary incapacitation resulting from impact by another vehicle. This (particularly from the rear or side) can cause the foot to rise off the brake pedal and even a still conscious driver can take appreciable time to overcome shock and bewilderment to process what has happened (even without airbags going off which in themselves are a violent event) and react. Lately I have been watching dashcam videos of accidents and (particularly in the US with the preponderance of auto transmissions) it does happen that stationary vehicles move off under power after being struck and they can and do move into cross or opposing traffic.
Of course the kind of more catastrophic incapacitation you have in mind such as heart attack or stroke or siezure is more dangerous at 70 and there is little that can be done about that apart from not driving at all. This is not a good reason to disregard a different risk which you
can reduce. Walking across the Motorway without looking is much more dangerous so it's OK to cross a 30 limit without looking?
As to the ultimately calamitous event which sees off the driver in a stationary vehicle, it is still safer for the other occupants of the vehicle, and pedestrians and other vehicles if the vehicle is not able to move off uncontrolled.