It's not just the cost of upgrading the workstations, it's everything.
moving away from windows 2000 means.
all the custom hardware (till keyboard, till screen, cash draw release, the little thing that puts the orders through to the kitchen) might not have drivers, and might have to change.
Indeed the POS software will have been custom written and might not work at all with a new version of windows (certain methods for example are now not allowed or work slightly differently).
far from just not impressed, you should read your contract for confidentiality clauses.