We find lots of non-alphabetic writing from earlier periods that does survive, though. Surely if they had alphabetic writing, we'd find that when we find writing?
Virtually all of the writing we find from earlier periods has been carved in stone. Without finding the rare monument or well-preserved grave stone, a traveler that arrived here 10,000 years in the future would find just as little evidence that we knew how to write.
I've heard this argument before, and generally don't buy it. It really comes down to how hard they're looking.
We have found many dinosaur bones that are hundreds of millions of years old. We have an Australopithecus skeleton (Lucy) from 3.2 million years ago. We have many examples of writing that would be similarly durable. Even if most things wither or decay or erode or get scavenged or build over, so 99% of it is gone in 10,000 years, there's still plenty that'd get buried and preserved. You give the example of monuments. They're not all that rare though. Every town has a few, usually right at the center, right where excavation would be most likely. I'm thinking of the world war memorials that every little English town seems to have, with the names of all their fallen soldiers. They're not ALL just going to turn to dust, right? There are temples in Egypt where you can still see not just what they carved in the stone thousands of years ago, but even the paint that they put on it.
So if we're all gone in 10,000 years and that traveller just buzzes by and doesn't scratch the surface or even look very hard, sure. But if they're excavating at the level that humans are today, it will be hard to miss that we had writing.