Scarcity still applies to robots/automation. Robots are not free, the resources used to make and maintain them are not free, and they still exist in a world where information and time are limited. Note that the fast-food-worker, definitionally, has another thing they're least bad at that they now move to - which is really the story we've seen with all of the process of automation.

Now, if you had robots that could replicate themselves indefinitely, with effectively unlimited resources, then yes, comparative advantage would cease to apply in the sense I'm talking about. But then we'd have a Star Trek replicator economy, and we wouldn't have to worry about not having an "income" from work.