No scientist is truly irreplacable, even Einstein. Eventually, someone would have done what Newton did. Same for Einstein. They just were smart enough and got born earlier.
I read somewhere that there is one insight Einstein had (no it was not e=mc squared) that others in time are not likely to have fathomed, and one thing Newton did, that others would not have grasped onto either, for a very long time indeed. What are those? I don't remember!