As for Observers and "dumbed down" explanations, there was no way they were going to fit everything about their motives/origins in this one episode. I'm guessing there's more to it than just "well, we screwed up our world in 2609; let's go back when times were a bit better". If we get a S5, then the Observers motives (among countless other things pertaining to this 2036 future) will probably be expanded upon.
If you view this episode as a simple introduction to this new setting, then perhaps you may think differently; though if there is no S5, then this will be the technical open ending of the series, at which point the lack of further elaboration will be unfortunate.

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