Customers really depend on their software to earn their living. When they for example can not send out invoices, they have a major problem. When they can not send invoices they will not receive money from their customers. When they do not receive money they can not pay their own bills. Such a situation could end in bankruptcy.

Do not think such a scenario is exaggerated. It is a nightmare for your customer. He will act accordingly. When you do not respond properly he will strike back furiously. You will go down under with him.