Wind and sun will always supply a base level of energy.

That is objectively false. The sun doesn't shine at night, and wind doesn't blow 100% of the time. So logically there is some amount of time that you do not get a base load provided only by sun and wind. Hence the need for storage at all. And yes it is a gotcha question, because it's something that anti-nuclear people hand-wave away as if the significant storage infrastructure to support a 100% renewable is just a rounding error, and not worth thinking about.