One might analogously ask: what would destroy our confidence in the modern theory of cultural evolution?
I think that "Digital watches in the Prehistoric" would have an effect broadly similar to Haldane's fossil rabbits in the Precambrian. With cultural evolution, such an observation might be a bit easier to swallow - due to the existence of another optimizing process capable of producing adaptations - namely human creativity. Nonetheless, individual creativity can only go so far. If advanced technology was present and the evidence of it was sufficiently good we might have to give up the modern theory of cultural evolution - and adopt an approach to history based on alien intervention - or some other source of apparent miracles. It would certainly mean a major rewrite of the history books.