What if the Observers were sent by someone (possibly Peter) to make sure nobody disrupted certain past events ? (think Terminator) That would be why they were seen on numerous times through the centuries, but why they should not interfere. Ever. For fear of damaging the continuity of the time line.
Say Walternate, amongst others, will, in the future, discover or create a way to travel through time, or at least to jump from one event to another. Given he is so focused on destroying the BlueVerse, he could be capable of trying to cause its end by changing some past events, where nobody could track him down.
What is unclear to me is when would he have taken possession of that technology, and where? I’m guessing in his universe, in a close future, given they are more technologically advanced than us. But now that Peter has created a bridge between the two universes, both futures will be different (he created another fork in the road, see my previous theory here). But maybe Walternate took that decision in the new future, and because he comes back so far in the past, both universes weren’t divided yet. So Peter was needed to allow the ‘new’ future to be created. Once again, see my theory about the grandfather paradox, where all the universes cannot exist without the others…
Read your old post Clem, I’m on the same page as you. And I like the idea that the Observers are terminator like figures traveling through time. One question: Peter may be the one, instead of Walter, because remember in Firefly he was able to use the Observer’s weapon. Unless of course, Walter programmed that way in the distant future. As to Walternate’s plan–we can reasonably assume there is a point of common history. When this is…who knows? Maybe it’s before where the wormhole opens up 250 mya. Anyway, the question becomes is the future of the series, the continued struggle between Walternate and his opponents?