SF is the art of the technical class. The central message is "You can fix it or create wonders by applying intelligence and dilligence to the problem."
Huh? That is not the central message of SF. That is one single theme used in some SF, and used in the most generic sci-fi out there. The conflict is man v. nature/technology or man v. society (or even man v. self), where the virtues extolled are up to the writer. Besides intelligence and diligence, some other virtues often key in SF include self-reliance, capacity for specific emotions (love/empathy/etc), having morals, willingness to deviate from the norm, etc.
Mainstream fiction is the propaganda of control of the general population: The central message is futility
What the hell kind of mainstream fiction did YOU read that was contemporary with Bradbury? In 1953, when Fahrenheit 451 was published, the books that topped the Adult Fiction bestseller charts were: The Silver Chalice (Costain), East of Eden (Steinbeck), Desiree (Selinko), Beyond This Place (Cronin), and Lord Vanity (Shellabarger). None of these books had a message of futility OR conformity; very much the opposite.
You are saying that Bradbury imported the mainstream fiction message of "Do what the authorities tell you to do. No matrer HOW badly they're doing and HOW bad things get, don't try to improve them. Anything you try will make them worse.". Not only was that not the mainstream message of the day, you would be hard-pressed to find that as a theme in any of Bradbury's works. I ask you to please name a single work of Bradbury's where this could conceivably be the case.