An issue here is: Is the kind of nanotech you're talking about even feasible? Even people with much more scientific knowledge than I doubt that.
First: What is this end-all-be-all nanotech that might destroy science fiction? Right. Von Neumann automata a/k/a nanobots, that is, nano-scale devices which
- reproduce in massive amounts like bacteria,
- cooperate in groups of several billion,
- follow instructions,
- produce their own energy.
Following problems have not yet been solved:
- How is a nanobot going to distinguish its products from itself?
- How is it going to store and process instructions? Remember you need at least one atom to store one bit.
- Will the solution be efficient? A billion of nanobots working on something will consume insane amounts of matter and energy compared to, say, a human with a sledgehammer.