^But that's part of the point no? It's a source of suspense, much like Hitchcock's "bomb under the table" scenario. Giving the audience information the characters don't have is a very old and proven way of keeping people interested.
Going back to Dune for a second, the real reason I found the newer books so tedious is because they're just a purely linear description of events. "This happened, then some thing else, and then...and then...and then....zzzzz." Unlike his son, Frank Herbert knew how to draw in readers, not with big events but with big ideas. Most of the original Dune books are really just a series of conversations and interior monologues that really brings the characters to life. Indeed, most probably don't notice it at the time, but for a series of books about the rise and fall of civilizations across thousands of years there's actually very little action. Most of the battles happen "off-screen", save for that one right near the end of Chapterhouse.
Likewise in B5, we're given a glimpse into the fates of these characters. We get to see the world through their eyes and it gives greater depth to the events of the "present" knowing how some things will go down later on. Yes, Londo will one day in the next few decades find himself joined to a keeper. At the end of 'Rising Star' we even hear from Delenn (presumably speaking from some point in the future) : "The next 20 years would see great changes, great joy and great sorrow. The Telepath War, the Drakh War. The new Alliance would waver and crack but in the end, it would hold."
But none of that spoils anything really, now does it? Given what we've seen, an open conflict involving human telepaths seems to be inevitable. But that doesn't make watching the events that precipitate it any less interesting.
Or to give an example closer to reality; the other day I watched 'Lincoln' for the first time. Did it matter that before I saw a single frame of film I knew that he would successfully see slavery permanently abolished, or that he would at some point have a one way trip the the theatre? Of course not. The same is true with just about any kind of historical drama where the backdrop is well known. In a sense, that's what the likes of B5 & Dune are. They're framed as historical dramas that just so happen to take place in a fictional future.