Spontaneously losing this energy would at least violate the law of conservation of energy in some way.
It attempts to solve the starlight problem by claiming that the speed of light in a vacuum was faster in the past and has since decayed to the value we observe it to be today.
Since the development of electronic digital counters and pulsed lasers, it has been possible to measure the speed of light in the laboratory with extraordinary precision.
Even if the speed of light reached the proximity of its final value decades ago, there would be enough residual decay as the value reached its limit asymptotically for our modern apparatus to detect.
There is none, forcing the proponents of the theory to toss out the exponential decay which governs nearly every phenomenon in the universe in favor of wild trigonometric functions they found by brute force curve-fitting, without an underlying explanation of "why".
Ultimately, even many creationists have abandoned c-decay.
Keep in mind that, in order to be useful for validating an age of the universe less than 10,000 years rather than more than 10 billion years, the speed of light needs to be more than a million times faster, a difference which would be difficult to miss.
We're not talking about a difference of a fraction of a percent.
Creationists would have us believe that the speed of light is arbitrary and somehow separated from the rest of reality; however, c is not just "the speed of light." It is a universal constant which is observed as unchanging no matter who is observing it.
It can be thought of as the speed which all objects fly through the four dimensions of spacetime — if you move along "space" you have less of the speed left to go through "time" and you experience the effect of time dilation.
"c" is also used in many equations related to electromagnetic phenomenon such as Maxwell's equations; and it is the fact that these equations mention c without asking what the speed is relative to as should happen in a relativistic universe that lead to the notion that c is constant for all observers.
It is also a key component in Einstein's famous E = mc equation; in this case, if c was larger in the past, then matter would have had more unit of energy per unit of mass in the past.