I must admit that I didn't quite grasp how Walter taking the boy to 2167 reset time and eliminated the Observers when I sat down and thought about it over the weekend. My original thinking was that 2167 should have been a future where the Observers had already been there for 152 years. However, the device Walter used to travel to 2167 was built by the Observers and we already know how they can step between different realities, even change things while still being aware of all possible outcomes, so I guess Walter and Michael jumped to a time when the Observers had not invaded in 2015; a future that stemmed from the original 2015 not the one where the Observers invaded. That is supported by the fact that the Observers were moving things from 2036 without changing their own future (it would create a paradox otherwise since their future would become one were they'd been around since 2015 with plenty of resources and therefore would have no need to invade the past).|
Very confusing to explain really but it kind of makes sense to me. I'm guessing the paradox Walter was referring to when resetting time was where he would exist as the same person in both (the reset) 2015 and 2167. It does beg the question though, given the somewhat rushed ending, of whether Olivia and Peter were aware of the alternate future that had occurred and what they thought had happened to Walter (did he disappear from their perspectives?).
One thing that doesn't quite make sense is that if the bad Observers were eradicated from history and replaced with good ones (with both intellect and emotions) then who saved Peter from drowning the lake? Donald/September explained that he and 11 other Observers were sent on a mission which they had no idea was the prelude to the invasion. So if the bad Observers never existed then there would have been no need for September and the others to have been sent back and, thus, there wouldn't have been anyone to rescue Peter. Or maybe the Observers not being around meant that Walter was not distracted by September and he, therefore, noticed the cure for Peter and he saved his original son?
Time travel alone is confusing enough but add in alternate universes where people have led different lives (did anyone else think it was a bit corny that Michael gave Walter all his original memories back?) and I guess it is best not to scrutinize it too closely.
PS3, PS4, PS Vita, Xbox One, PC (Intel Core i7-4770K, 16 GB, 3 GB NVIDIA GeForce GTX 780, Windows 8 Pro), iPad 4, iPad mini with Retina Display.