The DVDs are kind of a double-edged sword for Sci-Fi. On the one hand, they'll raise awareness of the series, and may get people interested in sampling it on Sci-Fi before they invest $75 or so in the discs. On the other hand, substantial DVD sales will probably cut into their audience - at least once all five seasons have been released. In the meantime it is probably more of a plus than a minus. I know a lot of people (I'm one of them) who became intrigued by X-Files after catching an episode here and there several seasons after it started, but didnt want to watch it the new episodes because they'd be picking up a complicated mythology in mid-story. We wanted to see it from the beginning. The DVDs made this possible. I bought the first season essentially "sight unseen" and really enjoyed it. When F/X started to rerun the second season, it became the only series I watched on that network. (This may sound like a very idiosyncratic response, but I've seen posts by a surprising number of people who bought the X-Files first season for almost the same reason on the Home Theater Forum. Similarly I'm waiting for the 24 DVD release, because the neither of the weekly showings was on at a convenient time for me, and taping is a pain. I've never seen it, but if I"m working when it comes out, I'll probably buy it.)
Something similar might happen with B5, especially since S2 will be cranking up around the time the S1 set is released. But even if the switch to 5 days a week is permanent, I wouldn't expect Sci-Fi to change the timeslot anytime soon. That is something that will only be done in response to an improvement in the ratings (or a sharp drop in ratings for Trek at 6 PM.)
We'll just have to see how things break.
Regards,
Joe