I've heard the same argument about Firefly--it was on TV too early. The show aired in 2002, and there was nothing like it on the major networks or even the major cable networks at the time, except possibly Enterprise--but even that was on UPN, not a "major" cable network like USA, TNT, etc. But look at the TV landscape today, and over the past several years: Lost, Smallville, Colony, The Walking Dead, Fringe, not to mention the BSG reboot...and many others. The quality of all these shows is debatable, but that's not my point--my point is that they were all on TV, and all (rightly or wrongly) for better or worse. And that's not even counting all the sci-fi and comic book awesomeness we've had in the theaters over the past 3-4 years. I've heard it argued before that if Firefly had premiered in today's TV landscape, it probably would have survived.
I would watch a reboot of B5. My issue with the show was never the story and plot (although some of the non-story arc episodes were a little silly). I just couldn't see past the poorly-made sets, bad special effects (I know it was the dawn of CGI, but still), and the poor acting on the part of some of the cast members.