The reason the speed of light appears to not have changed in the last thirty years is because that is the amount of time the speed of light has been measured via atomic processes. Since the speed of light is in sync with atomic processes, there is no way a change in it could be measured via those means.
As far as the old measurements go, a couple of points.
1. If it were simply a matter of observational/instrumental error, then we would expect to see measurements on BOTH sides of the current speed of light. We don't. What we see is, despite all possible errors, a steady decline in the measurements, even when taken by the same people with the same instruments over a number of years.
2. Other atomic constants have been measured as changing in exact relation to the changes measured in the speed of light.
It's not a creationist issue. It's a physics issue and it has been being discussed in secular peer-reviewed physics journals since Physics Review D came out with two articles on a changing speed of light in January of 1999.
So it cannot be foisted off on error or us stupid creationists, I'm afraid. The data does not care who sees it.