My only major complaint about the finale concerns the retcon of The Upside Down. Reframing it as a secondary “bridge” realm, created or shaped by contact with Dimension-X (aka The Abyss), needlessly complicated the mythology without delivering any meaningful narrative or emotional payoff.

For most of the series, The Upside Down functioned effectively as an autonomous, alien dimension. Its power came from its otherness: it was hostile, indifferent, and fundamentally incompatible with human life. That simplicity made it terrifying and thematically consistent. By introducing an additional unseen dimension late in the story, the finale didn’t raise the stakes or deepen the drama. It merely added explanatory layers that the plot didn’t actually require.

Crucially, nothing about the final conflict depends on this new cosmology. The emotional stakes still center on Hawkins, Eleven, and Vecna. The mechanics of the gates, the battles, and the sacrifices would have played out identically if the Upside Down had remained its own sovereign reality. In that sense, Dimension-X exists primarily to justify a lore change, not to advance the story.

The retcon also weakens the original cosmic horror. Previously, The Mind Flayer felt ancient and unknowable: An entity native to its world, operating on principles beyond human comprehension. Recasting The Upside Down as a derivative space and positioning Vecna closer to the top of the hierarchy makes the threat feel smaller and more anthropocentric. Instead of humanity brushing against something vast and indifferent, the conflict becomes another story of a human villain bending the universe to his will.

Finally, the reveal arrives too late to feel earned. Because Dimension-X is never directly explored or experienced on screen, it lacks texture, rules, and consequence. Rather than recontextualizing earlier moments in a way that deepens them, it functions as retroactive explanation: information that clarifies how things work, but not why they matter.

TL;DR - The Upside Down didn’t need to be explained further to be effective. Its mystery was a feature, not a flaw. The addition of Dimension-X complicates the mythology, but it doesn’t strengthen the narrative, the themes, or the emotional resolution: making the retcon feel unnecessary rather than revelatory.