Don't think it was claimed that they were never enforceable, not by me anyway, hc. The cases you cite were all examples of confidential information being used, or potentially used, by people in trusted positions, to destroy the complainant party's business or very severely affect it. Effectively, the defendants were stealing the business, or planning to do so, by using confidential information in a massive breach of trust. You wouldn't always need an express term in a contract to take action to stop that.
But it's difficult to see, prima facie, how a salesman, even a technical one, with no such information to sell or use, is properly subject to an enforceable term as broad as the one described (or at all). Salesmen do not normally have a 'following' either. Clients don't say " I was so happy to be sold X's product by him, that I must go and buy Y's product instead, from him now he's moved". You can imagine a car salesman selling Toyotas who moves to Ford. He'll probably be rude about Toyotas, and their technology, and praise Fords, when he had done the reverse but that's just how life is. and has to be accepted.