They don't connect to the company server. They just shut down and display a message to the operator. It's up to the operator to receive an unlock code from the manufacturer. So, there's no server involved. It's all human-to-human interaction.

But again, in 15 years all these codes will be lost by the manufacturer itself, so your main point is valid.

Um, you stop using the machine? The sort of business that is buying these CNC mills isn't going to be interested in using an out-of-support machine (too much risk) and likely depreciates the machine to $0 value in 5-6 years anyways.