Re:Good News, and Very, Very Bad News
Well, given that “Diane Baker” isn’t a current B5 charactor and is the mani charactor in the movie as written by JMS, I would argue that the movie was gonna do just what you say: have a new main charctor with the old cast in “supporting” roles. In fact, that is what would HAVE to happen given that JMS has said repeatedly that the story of Babylon 5 itself has been told and he won’t be revisiting it.
Recasting the original charactors is dumb, though. Better to have none of the original charactors in the film at all. I don’t think a movie needs big names to sell, though. It seems that The Lord of the Rings did just fine with a cast that was generally unknown before their appearance in it.
The reason for the feature film is to make the kind of money that a feature film can make, which dwarfs anything a TVM can make. Plus, with the virual guarantee of significant DVD sales (WB made more than $100 million on the sales of the series sets) made it seem like a no-lose situation. They seem to be rethinking the idea of distributing it themselves, though, so they may be following your line of reasoning.
I would have preferred a TVM myself, because it would be likelier to happen and would be in our hands sooner.