The speed of light in a vacuum is a physical constant. Like just about everything in science, it's taken us time to narrow down its speed but we have it worked out quite well now. Here's a brief history of experimentation and estimation. It was estimated to within 1210 Km/s of our current accurate understanding in 1726! http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html
Saying that our estimates of the speed of light have changed over time doesn't do anything to refute the fact that the light from M51 has traveled for about 23 million years to reach us. If you want to further argue in defense of ignorance I suggest using a better argument.