While that author's conclusions might hold true for past centuries, the advent of widespread, even global media distribution can be said to have slowed—or even stopped entirely—the phenomenon of language drift. Having to rely on memory turned communication of separated communities into a sort of decades-long telephone game, but now, most children are educated in their native tongue by someone other than their parents. That education is facilitated primarily through the use of products that use standardized language: school textbooks, toys from global corporations, and most importantly, broadcast television. As such, it is entirely plausible that humans a thousand years in our future could understand us today.
Consider English, for example. While Shakespeare's writing seems stilted and avails itself often of circumlocution, it is still understandable with a small bit of education. That's over six hundred years' difference, and though there have been changes, they are a far cry from "unintelligible." The changes to grammar, syntax, letter forms, and vocabulary that English has experienced are largely due to its widespread use in the majority of scientific fields, as well as its decentralized nature, but most of those changes have been small.
- Grammar and syntax: A slow movement away from strict (some might say blind) adherence to Latin rules.
- Letter forms/typography: Gradual standardization and the trend toward simplified typefaces (gothic > serif > sans.)
- Vocabulary: Lots of changes, but primarily the addition of words to satisfy new concepts and inventions, with the occasional word migrating from one meaning to another: intercourse, for example.
As such, I find it entirely possible that English could survive in an intelligible form several hundred, or even thousand, years into the future. My personal views are that we'll start mixing Chinese with English over the next hundred years, but that's another discussion.
Above all, though, the writer has the responsibility to make sure his or her dialogue is understandable to the reader. Even when you're breaking the rules of language, you have to do it properly, else no one in your real world will understand what you're trying to say.