Isaac Newton made some incorrect assumptions. In most situations on earth the error is small enough to ignore (you don't need to worry about time dialation to calculate the projectile path of a thrown rock), but there's depreciencies in the cosmos (like mercury's weird precession). So in a sense, elementary mechanics never was correct, but it was the best humanity had for awhile until Einstein's relativity and it's still useful in many not-extreme contexts.

Really, until we actually find dark matter, we can't say for sure that relativity is correct either, but that's just science.