People used to believe that the earth was flat. That was a wrong idea. That wrong idea wasn't held because people were mentally ill. It seemed like the right thing to believe at one time. But it was not true.

Some ideas vanish because people find them to be false.

Then there are ideas that are not a matter of truth or falsehood. At one time, lots of Americans thought it was okay to own slaves. That got bumped off by a newer idea that no human should be enslaved. Ideas matter.