So Deconstruction of Falling Stars is the next controversial episode? Hmm.
What about Mars, or the other human colonies? Earth would not be left alone to revert to the dark ages.
Isn't that exactly what the segment is showing? The rangers from other worlds and colonies are on Earth slowly re-introducing technology. The whole thing is shot from surveillance cameras they've installed in the monastery.
I don't think it's a pessimistic episode. The message is: yes, we're going to stumble and make more mistakes along the way, but in the end we turn out OK. Thanks in large part to people like Sheridan.
The fact that his legacy remains a force for good a million years later is huge!
As others have said, in the last segment we've essentially become the next Vorlons. However, since we're shown archiving the history of B5, we've hopefully learned from it, and won't make all the same mistakes. Just like every parent wants their children to succeed them, we get a little bit better and better each generation. Vorlon, to Human, to whatever comes after us.