It seems nobody cares about the parallel universe anymore, is it still there?
What would happen if the observers took control over there, too? Or could humans in that world defeat the observers and then help fringe team in original universe?
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At the end of 4x20, Worlds Apart,
The bridge between the two universes was closed, permanently preventing anyone from either universe from going to the other one.
In 5x04, The Bullet That Saved the World, Peter makes an annoyed remark to Walter:
"There has got to be a better option for getting us into that train station than a portal that lets us see into another universe."
So in theory, they could still use one of the various methods from prior seasons to travel between universes, but breaking the repaired barrier between them would leave them in a worse situation than in previous seasons: Both universes on a collision course where neither will survive, and the machine no longer exists/is no longer accessible to repair the damage.
In addition to Izkata's answer, it was confirmed in the penultimate episode of Season 5 that the alternate universe still exists when:
Olivia crosses over to rescue Micheal the child Observer, who had been captured and was being held by the Observers. We also learn that Lincoln from the original universe is still present in the alternate one, and that both he and Fauxlivia are aware of the other universe, as they greet Olivia warmly and as friends, indicating that they've met before.