Imma just break down why the ending fucking blows.
1) This movie - in the writing, the direction, the work of the cast - spends 99% of the runtime getting you to care about the characters and their situation, and does a pretty decent job doing it. The cast has chemistry, the tension is there, the sense of a threat is there. For a movie that's basically Pretty Hollywood Types vs Indestructible Space Squid it takes itself seriously enough to make you care, but not quite so seriously that it becomes a The Thing-esque lovecraftian horror that sets up the possibility that literally everyone is doomed. Everything up to and including Gyllenhaal and Ferguson's desperate final plan points to an eventual hard-won victory, where only one person survives, and, just for kicks, there's a remote chance that Calvin will someday encounter some other civilization or something and wreck havoc there. But, the point is, the movie doesn't just promise, but actively makes it look like those other five people didn't actually die for nothing.
... Except they totally did! So what's the fucking point of the entire thing then? The twist doesn't just add anything to the movie, it goes in the opposite direction and cheapens it. The movie spends 100 minutes playing fair, and then kicks you in the balls and laughs at you. It's exactly what a nihilistic 13-year-old who thinks he's way edgier than everyone else would think of - "What if it's like Alien or The Thing, except - get this - EVERYONE TOTALLY DIES LOL? And then some upbeat old song plays over the end credits! Like in Reservoir Dogs or something! Because irony!"
2) To make it even more pointless: taken just by itself, the ending sets up another cool movie... that no one involved has any intention to ever make. In the world of the movie, what must follow is humanity's desperate battle with an indestructible squid from Mars who's presumably only gonna get bigger and bigger. How awesome does that premise sound? It sounds like the best thing ever. But you're not getting that. What you're getting instead is a pointless feature-length prequel about how that squid got here, starring characters who try their best and still all die for nothing, with no reward or payoff of any kind.
I haven't seen a movie shoot itself in its pretty little head in the last two minutes like this in a long, long time.