Jones' plans become clear to Walter: bring the two realities together, so they will destroy each other, and therein engender a new universe, of which Jones will be lord and master. The only way to stop Jones from doing this, other than stopping Jones, is to close the bridge between the two realities. Olivia has a plan, but it fails. No one knows how to get to Jones. Closing the bridge is the only option.
We get wonderful scenes between Walter and Walternate like nothing before in the series. The other side will suffer with the connection broken - likely defeating Faulivia's hope of seeing rainbows again, but an other side even without rainbows beats no existence at all.
Peter stays on our side - "home is where the heart is," he tells Lee, and Peter's heart is with Olivia. But Lee's heart is with Fauxlivia - this was clear from the episode in which alt-Lee was killed - and Lee will stay over there. Our side will be even more like it was in the first two seasons.
And so the bridge is closed. Walter fears that Peter may not survive the closing, but he does. Walter says he'll miss them, and so will we, but there may be a way to reopen the connection, after Jones is taken care of (he may figure out a way to reopen the bridge anyway, for his own nefarious purposes).
And now we likely know at least part of why there was no other side in last week's future Fringe episode. The future with the Eternal Bald Observers is a future that builds on tonight's closing of the bridge.
But what can be rent asunder can be brought back together. Olivia tells Fauxlivia to keep looking up (for rainbows), and we can look forward to a two-hour finale and one more season for a science fiction series that has become one of the very best ever on television.
... Fringe 4.2: Better and Worse Selves ... Fringe 4.3: Sanity and Son ... Fringe 4.4: Peter's Back, Ectoplasm, and McLuhan ... Fringe 4.5: Double Return ... Fringe 4.6: Time Slips ... Fringe 4.7: The Invisible Man ... Fringe 4.8: The Ramifications of Transformed Alternate Realities ... Fringe 4.9: Elizabeth ... Fringe 4.10: Deceit and Future Vision ... Fringe 4.11: Alternate Astrid ... Fringe 4.12: Double Westfield / Single Olivia ... Fringe 4.13: Tea and Telepathy ... Fringe 4.14: Palimpsest ... Fringe 4.15: I Knew It! ... Fringe 4.16: Walter Likes Yiddish ... Fringe 4.17: Second Chances ... Fringe 4.18: Broyled on Both Sides ... Future Fringe 4.19