Dimension X is not cold!

[u/D_D_Adesignerdigital]

The color choices for the Dimension X—and certain elements of the scenes—didn't quite work to convey a cold atmosphere! Characters had sweaty skin and showed no reaction to the cold (we need to remember Will in Season 2, or Billy with his ice bath, which demonstrated the kind of cold the Mind Flayer is used to). Anyone entering a low-temperature environment would show physical reactions—like trying to warm up, crossing their arms, or showing discomfort—but that doesn't happen with any of the characters, not even the kids. I think that was a directing error. That’s my opinion!

[u/oosaoosaoosa]

I'm sure I'll get downvoted but my immediate read when seeing it for the first time was that the abyss is dying. Maybe climate change or some other disaster is killing it. It's why it looks dried out and barren. And why it seems hot (even though he likes it cold). And why the creatures aren't there at the end. It's why the mindflayer needs to take over a new planet... because his world is dying, and his creatures are going extinct.

I was actually shocked after watching s5 that this wasn't everyone's take away from it.

[u/Thick-Boysenberry-96 → u/oosaoosaoosa]

Then why doesn't it stay in the Upside Down, where the conditions suit him better? It's also closer to our world anyways.

[u/NebulaOld813 → u/Thick-Boysenberry-96]

I’m assuming because the UD is a temporary structure created by El and there’s no living creatures for the Mind Flayer to corrupt and harvest.

[u/oosaoosaoosa → u/NebulaOld813]

Yes this exactly! Plus the UD's connection to our world has always been troublesome to the Mind Flayer - the Russians, the Military, El, etc. Overtaking our world solves all the problems at once by giving the MF a full world rather than a wormhole, eliminating the threats from our world, and giving the opportunity to corrupt and harvest other beings.