The principle of relativity, which what OP was talking about, cannot be derived from Maxwell's equations alone.
More novel, from einstein, was GR but many insights here can be found "in the air" amongst professional physicists, in particular you could see GR as following from a formal elaboration of Mach's principle
(itself, in many ways, just a reply to a thought experiment Netwon had made centuries earlier)
The point is that "non-crank" physics doesnt have the crank-like "moments of genuis" that people assume. It isnt just plucking ideas from the air. Those ideas are heavily embeded in thousands of years of reflection
But special relativity? Yes, Henri Poincare had already come up with a mathematically equivalent theory to special relativity. Einstein's is a much simpler theory by all means, but the original commenter talking about it like it came out of no where and people must have thought Einstein was crazy or lost his mind, that's just a gross misunderstanding of history.
If anything, Einstein's theory of special relativity gained acceptance rather quickly because it basically converged to where physics was already heading.