That makes the story dismal, not the ending.
Also, in Soylent Green they weren’t (yet) facing a “nothing left to eat except each other” situation. Rather, Soylent Green was the only high-protein food ration available, the other Soylents being cheap mass-produced starch.
I was wondering how far I would have to go before I saw “Dear Zachary”, even though the plot swivel is nowhere near the end of the movie.
When I saw it, I only knew beforehand that it was about an international custody dispute.
I agree, we do see people - mostly the police - eating normal food, but we don’t know precisely how much is left. I mean, how long could the population survive on wafers?
Anyway, I never thought I get to say “*Soylent Green is made of people !!” outside of a joke.
They could subsist– miserably; but already there were food riots due to the limited supply* of Green. The book has the protagonist mention that his diet is deficient enough to allow for him to be scrawny and paunchy at the same time.
*a self-correcting situation? 
Definitely putting Come and See on the list…
It fits “dismal.”
'Night, Mother and Mother Night are a double feature of grim.
Kes (1970s Barnsley movie) ends with the one hope for its one character with hope being murdered because the boy bought chips.
Its sequal, Kesses, is very different in tone and often seen as an alegory for Britains involvement in the Vietnam quadmire
“Soylent Green Is People!” is something added to the movie; it was NOT in the book (Make Room, Make Room), which was pretty depressing anyway.
Self-correcting: Not entirely. One has to assume that even in this situation, they couldn’t safely use “roadkill” (people who were found around town, who’d starved to death / been murdered / whatever). At least in the movie, there were places you could go to commit suicide in comfort - presumably that was the people that got used for food.
Some additions to the list:
Cloverfield (the original; have not seen their sequels): you know the two main characters are about to die. And there’s a question as to whether the humans will be able to defeat the monsters at all.
On The Beach (how has nobody not listed this before? did a search and did not see it).
A very current movie is Santosh (2024), which is shortlisted for an Oscar nomination as Best Foreign Language Film. I won’t spoil the plot, only say it’s sort of a more depressing Indian version of Training Day, with a theme that is summed up by one of the characters:
“There are two kinds of untouchables, in this country. The ones that no one wants to touch. And those that no one has the right to touch.”
Anyone mention Rosemary’s Baby (1968)?
Even if she’s gone mad or it’s really real, the coven isn’t going to let her baby go.
Which is worse?
“Aren’t you his mother, Rosemary?”
“What have you done to it’s eyes!”
Miracle Mile
The two main characters have been trying to get to a possible evacuation site, and EVERYTHING goes wrong.
Oh, I mentioned On The Beach a little while ago. You cited that one already, obviously. No clue why a search did not find it.
Related…Monsters (2010)
https://www.imdb.com/title/tt1470827/
The movie is great but IIRC ends up being very dismal because you may or may not realize that the two leads…die. You actually see their bodies in the opening scene which is shown in video camera footage which turns out to be a flash-forward.But you don’t know who they are at that point so its just two random civilians. Kind of soured me a bit on the movie.
Plus (it was implied) those that got scooped up in the streets by the riot-control trucks.
The Revenant was a pretty grim and dismal movie from start to finish. Much more so than the actual story IRL apparently.
Mark L. Smith who wrote the screenplay also wrote the series American Primeval on Netflix as a fictionalized telling of the Utah War with the Mormons as the backstory.