"the whole genre (sci-fi), not being based in reality, is therefore implausible."
That's demonstrably false: Many sci-fi stories from several decades ago have now come true. People went to the moon, there are satellites in orbit (satellite orbit is named 'Clarke Orbit' after sci-fi author Arthur C Clarke who first talked about it btw), there are touchscreen devices and bionic limbs. There were no CCTVs when 1984 was written but was 1984 implausible? Not at all. It was very plausible - readers felt that it could happen, that kind of controlling state aided by technology was very possible.
Good sci-fi predicts the future in a entirely plausible way, with a credible story and characters. Bad sci-fi has a silly story that makes no sense, with holes you can throw a dog through. That is a big difference. It's the difference between Neal Stephenson's Anathem and Diamond Age, for example, and Iain Banks' Culture books.
Even from a simple linguistic perspective, implausible and non-existent are not synonyms 