They do have a good idea of what they would go to if they needed to upgrade tomorrow. It's just currently a no sale, supporting a new system will cost more at this time then the old system.
Can you say if that system would be any more maintainable than the current one, especially e.g. 7-8 years after installation? Do these restaurants update e.g. prices or sales tax rates without outside help? Could they radically change their menu if they needed to do so? Are there forests of sticky notes sprouting from the sides of the machines reminding users how to make the system do what they want? How long does it take for new waitstaff to use the POS without supervision?
Is the current setup PCI-compliant, and will it still be a couple of months from now?