Whether the cause of a propulsion failure is motor, prop, human error, or ESC matters not, just as whether the cause occurred during a prior flight or the present one does not matter. When you lose a corner on a quad, you're going down. Single engine planes become controllable gliders, twins have the ability to either continue flight or be selective with where and when they land. A quad has no options. Aviation is all about being prepared for the unexpected, and system redundancy is all about being prepared to deal with the unexpected. Don't discount having the ability to continue safe flight because it hasn't happened to you. For many it has happened and those that could continue flight still have a functional aircraft, and camera.
When you plan an operation around the law of minimums, it requires but a minimum number of failures to disrupt the operation.
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