There probably is an upper bound on down time, by which point the business has suffered some irreparable harm. It might not result in the business simply folding, but might result in significant expense or legal complications, long-term reputational damage, etc. In business continuity speak, that's the "maximum tolerable downtime," and while I don't know how Delta defines it for the impacted systems... I imagine they're not happy with how long they were down.