TwoSix
Everyone's literal second-favorite poster
I think this is the important point. If the game rules wanted you to view characters with some low stats as obviously unsuited to adventuring, why would they let you generate those stats?And single digit ability scores were never intended to be so debilitating. The game rules wouldn't have allowed you to have a 3 Int if a 6 was meant to be non-functional as an adventurer.
The problem is a pretty wide swath of the player base took that "Int * 10 = IQ" as actually meaningful, and thus a lot of people do actually think that anything below an 8 is actually an extreme disability. I think that attitude also combined with the idea of "dump stat = munchkinism", and thus the idea that any low stat characters are problematic got baked into the collective consciousness of the player base.
And thus you have the norm today of no stat can actually be lower than 8.