I like the literacy explanation. If reading letters was hard enough for enough people, why add extra complexity by introducing a subtractive system? But as the populous becomes more educated and there’s less concern about reading ability, it makes sense that the next domino to fall is “boy those 4 i’s in a row sure are hard to distinguish”. Complexity gets progressively introduced — that to me sounds like lots of tech adoption curves.
> ...the Roman numeral system changed to the more familiar subtractive notation. However, this was well after the fall of the Roman Empire.
This contradicts examples that Wikipedia has of subtractive notation during the height of the Roman Empire (though it's not clear to me when "IV" became the accepted standard form).