I think I recall Walter explaining the many-worlds interpretation at some point, but I guess the writers thought that two universes was as much as viewers could handle. If reality really does require anything that has a finite probability of happening to actually happen in at least one universe, I don't see how the Observers could shore up their temporal dam.
Anyway, the Vulcan Science Academy has determined time travel is not possible. Well, I assume they mean time travel into the past. I'm travelling into the future all the time.