Nah, dark or grim sci-fi were aplenty, many of them were just regular action movies (also popular then) but done in sci-fi setting.
I'm using Star Trek as a measuring stick, particularly TNG and later, because this was a very big, popular and long-running franchise, that happened to have baked in the idea of optimistic future for humanity as a core part of its setting. It's a measuring stick that spans almost six decades now, and you can see in it how sentiments changed over that time, and how the hope and optimism eroded.
Various dystopian shows and movies that appeared in that time, they were pessimistic by design, and thus work as spot measurements of what people thought would sell best at a particular time. Star Trek wasn't - the weight of an established franchise meant it could've kept selling optimism even as others would find it too financially risky; so Star Trek going darker measures a deeper change in the mindsets of writers and studio executives.