wouldn’t it be impossible to unmix any pigment color combo? And wtf does that sort of metaphor even mean?

It’s an example of a situation where it’s easier to start fresh than undo past actions, which by your point you show you understand.

I don’t think it’s acceptable to call the planet an unfixable mess.

Let’s differentiate between OUR planet, and the planet depicted in the movie. Are you saying that there are no ways in which a fictional future earth is unsalvageable?

Do you also rally against movies set in, for example, a dystopian cyberpunk setting due to not liking the scene it was set in?

There is no real response because we’re talking about a fictional future, with unknown ailments, established by maybe 20 minutes of film as a backdrop. They wanted to tell a story titled “interstellar”, not “terrestrial”.

Given all those unknowns, it stands that there are times when starting fresh is easier than undoing. Trying to unmix brown pigments comes to mind.

You asked:

If earth is in a climate apocalypse, and we have extremely advanced technology that lets us bring life to far out planets, then why are we leaving earth? Can’t those same technologies be applied to saving the earth people?

This is a potential answer, given the lack of established truths in this fictional universe.

It can be easier to start with a fresh slate than it is to salvage a mess.