B5's final seasons were on TNT weren't they? And they kept it going to the end of the planned 5 seasons. The B5 spin-off was excruciatingly bad and deserved to die.
Sci-Fi has been pretty good to Stargate, and it's a good idea to concentrate on Atlantis, which has a chance of some staying power if they keep away from braindead 'all that outer-space stuff was just a delusion and you're actually in a psychiatric hospital' junk - I'm looking at you Carl Binder. Certainly SG-1 season 10 picked up a lot from season 9, but all shows have their life-span. I'd far rather see SG-1 go out now before it turns into a zombie like NYPD Blue has.