There are many beliefs that people will arrive at through armchair theorizing, but only until they are corrected. If you came up with the idea that the Earth was flat a long time ago, nobody would correct you. If you did that today, someone would correct you; indeed, society is so full of round-Earth information that it's hard for anyone to not have heard of the refutation before coming up with the idea, unless they're a young child.

Does that count as something arrived at through armchair theorizing? People would, after all, come up with it by armchair theorizing if they lived in a vacuum. They did come up with it through armchair theorizing back when they did live in a vacuum.

That's why there are tons of historical examples and not so many modern examples. A modern example has to be something where the refutation is well known by experts, but the refutation hasn't made it down to the common person, because if the refutation did make it down to the common person that would inhibit them from coming up with the armchair theory in the first place.

(For historical examples,

1. It's possible that the refutation is known by our experts, but not by contemporary experts, or
2. because of the bad state of mass communication in ancient times, the refutation simply hasn't spread enough to reach most armchair theorists.)