In the final few episodes of Fringe, we find out that Walter & September's plan was to effectively change the future so that the Observers/Invaders never exist.
Olivia gets excited at the prospect of seeing her & Peter's daughter again, and indeed at the end we see them enjoying the day at the park, this time without Observers turning up and killing people.
But if the Observers never existed in this timeline, Peter and Walter would never have gotten pulled out from the ice.
Or, if you count the Observer accidentally distracting Walternate away from the cure for Peter (the cause of our Walter crossing over in the first place) - Peter would never have been taken to our side in the first place - i.e. he never would've existed on this side to meet our Olivia.
So my question is, Why does Peter Bishop still exist in this timeline?