More conventional forms also, to some degrees and in some times and places.

Many of the ancient people's of Mesopotamia leave little or no trace of their languages. Hebrew, an iron age language, leaves only scraps dating to the iron age. One of 2-3 complete sentences from the whole kingdom of Judah is an inscription commemorating the completion of a waterworks project for Jerusalem. They wrote it on papyrus and it does not survive.

Neighboring iron age cultures are known from single artefacts.

The Easter Island script was seemingly written on banana leaf.

There is no deep archeological record of knotted strings.

Speculation from a sense of evidence is trick. Personally, I think writing and proto writing existed and was invented and lost many, many times. Thousands maybe.

Also, while impressive... A scaled up in advanced literary culture is not necessarily necessary, or even useful.

Egypt for example, are those Monumental temples and pyramids, mortuary hieroglyphs and what not necessary? Useful?

It's hard to distinguish between the outputs of the civilization, and its objective needs. There may have been many ways of doing complex, large, dense civilization.