@RSE: Actually it's not the speed of light that's upper bounded (and note that as you correctly point out, the speed of light is not a constant, except in an ideal vacuum and no such beast exists). So it is fair to say the speed of light is never constant.
What is upper bounded (and again not constant) is the speed of signaling, i.e. information transfer. And I would not really want to live in a universe where the speed of signaling is not upper bounded. I can't even imagine what such a universe would be like. The havoc it would play on causality alone makes me shudder.
It could of course be argued that the upper bound is not the speed of light, but such an upper bound must exist. And I somehow doubt that the upper bound, if higher than c will be high enough to allow for IST.
To me the only semi-hopeful way that interstellar travel can be achieved is by space-time distortions that temporarily alter the topology of the universe to connect remote regions. Wormholes for the layman
Unfortunately, I know very little about General Relativity, so I cannot really comment on the feasibility of that as a practical means of travel. Which is probably why I find it hopeful
