Except that… I don't think Henry created the bridge.
The Upside Down is a massive wormhole that appears as Hawkins in 1983 with Brenner's researches on wormholes and exotic matter in it. It seems it was Brenner that placed the big red ball of exotic matter that keeps the wormhole from collapsing.
From what I understand:
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The Upside Down is a bridge that appears exactly as a small town for some reason.
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It seems to have been created by Eleven with Brenner's instructions.
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It is maintained solely by Brenner's exotic matter during all these years.
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Then, for some reason, Vecna and the Mind Flayer invade this bridge but are too careless to post guards on the one crucial thing that prevents the bridge from collapsing, thus compromising their multi-year plan (this big red ball on the very center of the Upside Down they clearly ignore even when even when a teenager shoots it with a 12-gauge caliber). A multi-year plan that wouldn't even exist if someone they don't control hadn't invented billion-dollar technology and placed it solely to keep a bridge open.
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Somehow, nobody knows about this big ball of exotic matter (no one from Owens' team in Season 2 thought to investigate… I don't know… the roof of the Upside Down lab in which their portal leads?).
For me, this big red ball of exotic matter is the biggest plot hole of the whole season, even with this short list:
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There is no power-cancelling device in the real world when Vecna attacks, despite the military clearly setting a trap by keeping the children there. I mean, the scene is gorgeous and all, but makes no sense if you think about it.
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Somehow, everyone forgot about the five (or more) pregnant women who were in the base, and the good guys' plan, although aware of their existence, makes no mention of them, so should we assume that no one evacuated them?
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There is no one to guard “the tree” in Dimension X, even though Vecna has already lost because no one was physically guarding it, and that is really their ultimate plan…
It really looks like they rushed something at the last minute so they could close the Upside Down and all the plots quickly. There are very good scenes in this season, but as you said narratively it's mid.
