It's true though, as human (+ tool) gets smarter, human (without tool) tends to get dumber in the domain the tool augments.

The question is will we one day have tools so powerful, that the human is vestigial, and tool (without human) is just as powerful and cheaper than tool (+ human)?

True in general, yes, but writing is an elegant solution where the longer something is, the more likely you are to write it down because you are less likely to be able to remember it. The shorter something is, the less likely you are to write it down because it’s a pain to get out your scratchpad (or iPhone) for ten words or less.

Yes writing can only augment human intelligence.

AI might replace it.

I do think it's murkier then that.

Writing "replaced" some forms of intellectual effort.

And it's yet to be seen how AI will play out.

This is the case today for ai chess engines

But there are still people playing chess professionally because playing well is not enough. (You have to entertain an audience, which is only possible by "just playing well" if you're human)

It’s also an early example of the medium being the message. Socrates and his interlocutors are sometimes a parable on the transition from oral to written culture.

I find that it is true that new mediums and new technologies for language have a numbing effect on certain senses and an amplification of others.

Writing is beneficial in one regard but does have an impact on memory. Epic poems of great length were memorized in their entirety, a skill that would be a lot easier to develop in a world without writing.

Personally speaking, I've got much worse at stripping tree bark off with my teeth since I evolved opposable thumbs.