Totally fair skepticism. If you wanted to make it last just 30 years (standard early-retirement duration) then sure, I'd agree with you, probably even 4% would be totally safe. But 60 years? Too many things can happen, I have to be super conservative before taking the decision (forced or not) to pull the plug, hence my estimation.

Again, quite possibly I'll die much younger without even enjoying any of that freedom :-)

A thing to also take into account is that it is very unlikely you will live the rest of your life without going into some other money-making venture. People who know how to accumulate money tend to accumulate money.