The fact that nobody expressed the idea before fifty years earlier suggests that the idea is pretty hard to come up with independently, because had it been easy, people would have been coming up with it all through history.

This can be true, but it is also possible that an idea may be hard to independently develop because the intellectual foundations have not yet been laid.

Ideas build on existing understandings, and once the groundwork has been done there may be a sudden eruption of independent-but-similar new ideas built on those foundations. They were only hard to come up with until that time.