Tough customers, once won over, can prove to be extremely loyal. And in this case, the customer did Squarespace a favor by pointing out a flaw in their customer service approach. Undertraining can in fact be a systematic failure.

Unless the customer is causing you a major amount of aggravation or costing you money, you shouldn't be happy to see one go.

Also, FWIW, I don't think citing a comment in which the commenter calls the OP an "egotistical bitch" twice is good support for an argument.