Well knowledge wasn't spread as easily, and could be lost. People knew, then they didn't and so on. It doesn't detract from my point that flat earth is an example of an idea that died off when more people learn about it and scrutinize it.
> In the end, slavery wasn't abolished by debates: it was abolished by literally sending troops to shoot slave owners.
Abolishment of slavery took many angles since it was a massive change of society. Sure, you can pick one country where it required a war. But that doesn't mean the battle of ideas aspect is any less important. Even to rally troops you need to convince them their ideas are worth fighting for.
> (communism) still accepted widely enough to turn Russia into Soviet Union
Sure, and then we saw the results, and the bad idea died off (mostly)