Premise: They want to squeeze all the value out of your knowledge of the legacy system and then let you go. Assuming that's true, there is nothing in principle

Premise: They want to squeeze all the value out of your knowledge of the legacy system and then let you go. Assuming that's true, there is nothing in principle wrong with this. Just make sure you get compensated well enough to make it a win win. You deserve a premium for not getting to learn newer systems through your work.