He discovered the theory of Relativity which is 1 of two pillars of modern physics
Theory of Relativity - explains how to interpret motion between different inertial frames of reference and also places that are moving at constant speeds relative to each other.
Einstein discovered a new opening to physics with the Theory of Relativity
If he hadn't had discovered the Theory of Relativity then physics would be in a rut because it is a big part of physics. Almost like a puzzle piece, you can't leave one part of the puzzle out.