> The further back you go in Indo-European languages, they don't have articles.

Heh, while we're discussing English, this annoys me.

"The further back you go" is a continuum, the effect you describe should be too.

> The further back you go, the fewer Indo-European languages have articles.

Or match a discontinuity with a discontinuity.

> If you go back beyond a certain point, Indo-European languages don't have articles.

I actually meant it as a continuum. In classical latin, ille and unus exist, but they are not articles. In the vulgar latin period circa 800ad, they are pretty close to articles. In modern romance they're articles.

You can't say "beyond a certain point" because what point that is varies with geography.