"The speed of light" is very similar to the zero mass concept. I like to call it "the limit of motion of matter." No mass truly travels at that rate, but rather light is so small in mass that it is perceivable as traveling at essentially the limit of motion of matter to such a degree that we have actually labeled this limit to be "the speed of light".

As far as acceptance of photon mass, zero mass is akin to saying no mass; there is a fine line between zero and "perceived to be zero", but that fine line is completely disregarded or unconsidered in science which leads to photons being interpreted drastically differently from particles that are said to have a mass.

Also, regarding the "speed of light", when a vacuum is involved, it is much the same issue: even if it approaches a vacuum, it is impossible to truly be an absolute vacuum (such as absolute zero Kelvin) as there are infinite layers of smaller and smaller particles that will still make up the space. It is possible that under much greater vacuum conditions than we can create particles can travel in ways not fully understood (in other words, infinite speed rather than what we consider the speed of light), though that is something I don't have much else to say about, just a thought I come back to here and there.

Also, when there are infinite smaller layers considered, if we could zoom in on the lowest energy photon we could ever detect and its surroundings, it would be like looking at the observable universe. Those particles radiate in the same way as the stars and planets radiate, and their radiation may be so small that we simply are completely unaware of it. But we would see these "stars" as traveling at what we call "the speed of light", when the particles they emit would be traveling much faster just like the light coming from our stars relative to the stars' motion. If we keep going down further and further into this structure, perhaps again it is possible for particles to travel at what we may consider "infinite velocity" but they are so small that they are undetected completely. Just some extrapolations I like to ponder.