I think the worst assumption to make is that something doesn't or can't exist based on what we presently know, think, or are capable of doing. There was a point in our history wherein they thought that breaking the sound barrier was impossible. They reached what they called the "wall" and couldn't get past it. with some redesign of wings, body and tail and a good rocket, not only was it possible but it was smooth sailing from then on and through 2 to 6 times the speed of sound. The sound barrier on land was just broken a few years ago.

One day perhaps we will find throught the manipulation of energies that we can circumvent the critical mass theory and break the speed of light.