> They bought (presumably for money) a product, and expect that product to keep working as long as they need it to!

This depents on the terms of the contract. Typically, termination of service is covered in a license. If the license terms are okay in the respective jurisdiction, there is no fundamental ethical obligation to run a server beyond that. There might exist specific cases where it would be inappropriate to follow the terms by the letter, but that also has its limits.

Contract terms usually define legal obligations, not ethical obligations. They create duties parties must perform or face legal consequences--they don't speak to what those parties should do ethically.