[Virginia-American]

Einstein was not thinking in particular of the propagation of light as a means of measuring occurrence of events. He assumed the propagation of light as his standard for defining time only because it was the best known and steady such process.

Einstein deduced the constancy of c from his assumption that physical laws do not depend on the inertial frame. Assuming this, if you measure the constants that are used in Maxwell's equations you will get the same values no matter how fast you are moving. Then Maxwell can be used to calculate c.

[RightWhale]

The speed of light was not the point of the ST. It was a mere example.