Thoughts on the conclusion [Major Spoilers]

And live a lie that was dependent on alt-Helen never finding out this imposter (whose henchman killed her real husband) had taken his place and stolen his life? No. In just a couple of days he'd already put a huge strain on his alternate's marriage. She could sense he was different, it would never have worked out for long. The other John put himself at risk to help a friend. Something he wouldn't do - as we saw in his flashbacks, he could've opened the latch and no one would have been the wiser but he wouldn't stick his neck out.

If I had to guess, those people coming through the portal were refugees. They looked to be ordinary civilians, some of them children. Nuclear war, plague, asteroid impact, or whatever else, made them desperate enough to evacuate to a parallel Earth. It seems weird that they showed up just as the facility changed hands and the Nazis were gone though. There's not going to be a season 5 so they really should've given us some kind of backstory about these people. Who they were, why they were coming. Surely one traveler could have stopped to chat on his/her way by!

Actually, I was expecting either an advanced world to come through and render aid against the Nazis, or some kind of higher interdimensional life form would reach out, bring Juliana into that vision world and assume the form of Tagomi to communicate with her. She said "something's coming" right? But I suppose they did foreshadow what actually happened when she saw all those figures in her dream, approaching in the distance.

They were originally supposed to go for two more seasons after this, but three seasons ended up becoming one. I suppose we should be grateful Amazon told them before production began that four was going to be the end. The abruptness of both the Reich and Japan losing control of their respective empires was an artifact of that accelerated timetable.

They should've taken more time to explain the coup in Berlin. We knew John Smith and his friend Wilhelm Goertzmann were up to something but it was a shock when they took out Himmler's entire leadership cadre and seized control. They would've had to turn numerous bureaucrats and lower ranking officials to their cause in order to cover themselves, blame some radical group or other, and make it legit. All we know is, Smith tells Himmler he's never thought of him as a father, just a failed chicken farmer, then kills him with poison gas that's been placed there for the purpose, and when he goes outside everyone else is dead. Almost all the planning was glossed over because there wasn't time.

Ditto with the number of American officers ready to turn on a dime. There was no time to explore that in depth. In all fairness it's not hard to swallow, but collectively this stuff had kind of a rushed feel to it. And - again - the complete lack of explanation for the Portal People after foreshadowing their arrival from the season's first episode, that I found frustrating.

It would make no sense that every other world suddenly ended, I think that "end of the worlds" with the crumbling John Smith statue was just meant to be catchy. I gathered this was an evacuation from a particular parallel world that suffered some kind of apocalyptic scale disaster. Juliana's dimension happens to have been the site of extensive genocide which left a lot of vacancies. If you were planning a mass exodus you'd locate such places, which could accommodate large numbers of your people. They may have sent groups out to several other worlds too.

At least this is what I assume is happening. They never explained it. I hope the producers will eventually reveal who the portal people are in an interview, at a convention, on their social media accounts, wherever. It's not technically a spoiler anymore since the series is over.

Lots of people die in wars. Organized genocide just made things that much worse. This group coming through the portal seemed like a random sampling of civilians off the street. All ethnicities, all ages (including children), no obvious soldiers or scientists, which I why I assumed it was a refugee caravan, fleeing a post-apocalyptic scenario where their own world is no longer suitable for life.

Because they compressed what was supposed to happen over three seasons into one, there was a definite hurried feeling about it all. The Japanese and German control over their respective slices of North America fell apart very quickly. The coup in Berlin was sudden and not set up for viewers as extensively as it should've been. Taking out the Führer and all his close advisors would've required a lot of planning. The idea that American Reich officers would be ready to revolt when they saw a real chance isn't surprising. As one of them pointed out, they now had quite a few nuclear weapons based in their territory which would make them a world power in their own right. Germany could be stalemated (good old mutually assured destruction) to keep them from re-invading. It makes sense, it just happened very abruptly given the much more gradual worldbuilding of the first three seasons. Oh well. At least Amazon gave the producers advance notice, so they knew they had to wrap up the story.

You could take this as a glimpse of another reality that's also a warning not to trust him. There may not be anything more to it than that. If an alt-Joe was intended to factor into the story at some later point ... they just ran out of time.