I think you're right that the client won't care with the current state of things.
What if some other method of Foo needs to use a method that is on List but not on Collection? (Quick: To the JavaDoc! Are there any?) What if some refinement of the requirements reveals that Set would be better than List? (Back to the
doc! What requirement would do that?) These are two possible changes that would lead you in different directions. Design is often about guessing what changes might come and avoiding being stuck when they show up.
How would a future change from Collection to List affect existing code inside Foo? From Collection to Set? From List to Set?