Can a single human being reliably and robustly maintain a safety-critical system alone under any circumstances, ever?
Ever?
Ever?
Any system that requires perfect performance from any one human will fail.
I think the better question is how you get a system in which people are only responsible for any one facet to get the same performance out of people that a painter can get out of himself when he's setting up his own ladder that he personally has to climb on.
A painter who does a bad job setting up a ladder is going to have a bad time, a lone ATC operator having a heart attack potentially puts multiple large aircraft full of people in danger...
Mandatory scaffolding for roofing contractors would save some amount of deaths/injuries (and the related expenses) but add expenses to each job.
Some roofing firms refuse to operate without scaffolding; you pay for it or you find someone else.