There's one major problem for devotees of Einstein's theories. Einstein, being a scientist, defined his frames of reference; those things to be considered as given in order for the theory to work. A constant value for 'c' to all observers was one of them. So, 'c' is a constant for all observers simply because the theory requires it to be so. Subsequent experimental testing confirms that the theory explains the physical world better than Newton's Laws, while conveniently confirming Newtonian physics for the everyday velocities experienced by human beings. Relativity moves on to new technology, in applications such as satellite navigation, with corrections for relativity effects routinely factored into GPS navigational computations.
The problem now is in the observing, since all human observers live within the framework of space and time. For any development to the next stage, (as when Einstein stood on Newton's shoulders so that he could see further) a new frame of reference must be developed. Effects that seem to be beyond Relativity are already being observed. Further advances in physics will come from explaining these events. Personally, I'm not convinced that the velocity of light cannot be exceeded, though I don't see how super-velocities could be observed. But then I'm not a Particle Physicist, just a humble avionics technician.
Nevertheless, its interesting that even humble instrument bashers like me need to understand at least a bit about relativity as part of my job. Everyday acceptance and use is the driving reason for scientific research. No matter how esoteric it may be at the outset, good science eventually becomes practical. The mind boggles at what might come from chaos theory. Oh! I forgot! The weather forecasters already use that to help predict the movement of storms.
Mark my words. 'c' is NOT a constant in the REAL universe...
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Through difficulties to the cinema