Einstein's Happiest Moment: The Equivalence Principle

From a new paper on Einstein's Happiest Moment: The Equivalence Principle:

Einstein’s happiest thought was his leap from the observation that a falling person feels no gravity to the realization that gravity might be equivalent to acceleration. It affects all bodies in the same way because it is a property of spacetime — its curvature — not a force propagating through spacetime (like electromagnetic or nuclear forces). When expressed in a way that is manifestly independent of the choice of coordinates, this idea became General Relativity. But the ground for what is now known as the “equivalence principle” was laid long before Einstein, affording a fascinating example of the growth of a scientific idea through the continuous interplay between theory and experiment. ...

The earliest hints of something like equivalence came from Aristotle. ...

The EP can be said to have originated with Newton, as did experiments to test it.

So why would Einstein's happiest thought be a simple regurgitation of a principle that had been accepted for centuries? Wouldn't he be happier about unifying space and time, explaining Michelson-Morley, or quantizing the photon? The story is bizarre.

I have a theory about this. Einstein was proudest about relativity, but as far as I could determine, there was only one part of the theory that was original to him. He figured out in 1907 and 1911 papers that gravitational potential affected clocks. He did it by using the equivalence principle to relate it to a non-gravity scenario, where special relativity could be used to relate the clocks. I do not think this occurred to anyone else.

All the rest of relativity he stole from others. The gravity effect on clocks was his happiest because he could genuinely claim the credit.