Nothing is ever proven conclusively in science so lay that idea to rest. But to really address this point, there is a mountain of evidence which points to this "cosmic speed limit." We aren't assuming because we don't understand, we actually understand quite well.
Yes, we know amazing new discoveries can shake the foundations of science and challenge what we know. However, it's usually wise to wait until that happens. You shouldn't take that as free license to attack parts of modern science willy nilly.