This is a good topic for discussion because almost anything you can say about it is likely to be disagreed with in different ways by practically everyone. Who are the founders of SF? Who are the founders of civilization? The first question is probably as easy to answer as the second one.
I've always liked hard science fiction. I've always disliked fantasy. But I began reading SF in the mid 1950's and what was hard SF then would be considered fantasy today.
A really good example is Alfred Bester's "The Demolished Man", about a future time when the police can read the citizens minds and arrest those who plan to commit crime, thereby completely eliminating crime. It's the story of one of the richest men in the world and how he plans to murder his chief competitor, without thinking about it, ever, of course.
Today a story like that has to be fantasy. There's no scientific basis for mind reading and none was postulated for the story. It was just assumed that at some point it would become part of our evolution. So when it was written it deserved the name "hard science fiction", at least in a vague sort of way. By the way, it's very much worth reading. And it was the first novel to win a Hugo award.
I think I have to agree that the golden age of science fiction was when Heinlein, Asimov and Clarke were writing. I even have to grudgingly agree that Heinlein wrote hard SF, although much of his later stuff was really sexual fantasy disguised as SF.
It's also worth pointing out that in those golden days the general public considered science fiction to be a bit lower than comic books and barely better than porn. There were no genres in bookstores then. There was a section for fiction, in which all sorts of books, westerns, mysteries, mainstream, etc. were stacked together, alpha by author. Even fantasy and horror novels were there. But a lot of stores had a section hidden away from general view for science fiction books. They didn't want the public to see it. It might hurt their reputation.
That really didn't begin to change till Star Wars. Even 2001 didn't have the effect it might have because, even though nobody could deny that it was SF, nobody really thought of it that way. It was something of it's own.
After Star Wars SF books began selling a lot more and a lot of new writers entered the field and changed it drastically. I lost interest a few years later. I still read some of the older books from time to time but I've only read a few SF books written after the 1980s.
There were some other writers during the golden age that had a big impact on SF even though they weren't really SF writers. John D. MacDonald and Kurt Vonnegut to name a couple.
By the way, a little off topic but fun anyway, for any of you Bonanza fans you might want to search for some of the short stories by Hoss Cartwright, sometimes known as Dan Blocker. He was an avid SS writer and a lot of his stories had a nice SF flavor. They were really good stories as I recall. I read quite a few of them back in those days. I'm not sure where to find them today.
Barry