Relativity was actually first postulated by Galileo. What Einstein worked out was General Relativity and Special Relativity.
General Relativity was already being worked on and Einstein simply helped firm it up.
He actually got it wrong twice and had to publish new papers with math corrections.
At one point Einstein actually had it right, but the math was in conflict with General science believed in. Einstein's math showed that the Universe was expanding and under the views of Astronomers and Physicists this was not correct. So Einstein changed his math to fit the model.
This was a huge mistake as several scientists would later prove with other math that the universe was expanding and then Hubble showed the red shift in the spectrum which made a proof for this.
Einstein's primary work, however, was in the concept of space, time, mass, energy, gravity and light.
Einstein postultaed the mass grows smaller and time slows as you speed up. The time dilation was proven with dual atomic clocks.
Einstein viewed space as a kind of fabric and that this fabric can be distored by gravity wells. The larger the gravity well the more distortion.
That this distortion would cause an apparent visual shift, which was established during solar eclipses when stars visually close to the sun's disk were not exactly where they were supposed to be due to the gravity force of the sun, which is massive.
Einstein postulated the the speed of light in a constant in all situations and nothing can go faster than light.
It is hard to say what the others would have postulated at a later date.
It is even hard to say what happened with Einstien.
He and others were actually in serach of the holy grail of science. The unified field theory and it is possible that these pieces came from Einstein's work on the problem.
In order to unify the concept of how things work one has to thoroughly understand each concept. We are dealing with magnetic waves, radiation, photonic light emissions, gravity.
Einstein's work deals with all these factors but it doesn't present a common thread.
It doesn't stipulate that
WAVE=x2(y3)
Or
Gravity = Magnitism/Radition
This is the kind of glue they were looking for.
They were looking for either an algebric, trigonimic or caculitic way of describing how all elements of physic inter relate.
A whole bunch of them has pieces of a puzzle, Bohr, Planck, Heisenberg, Newton, Einstein and others.
But there was no comprehensive way to explain how the puzzle fits together.