Strange how language drift seems to rarely happen to most authors' native tongue... 
.
But then, reading a language, even your own, after many centuries of evolution, would prove quite the straining experience when one merely want's to read "pulp military sci-fi" to escape for a while.
Real world example. I bought Marguerite de Navarre's "Heptaméron" about a year back. My interest had be piqued at the highest point after reading a few of the short stories contained whithin while I followed a course in french litteracy in the 16th century. I tried to read it, I really did, but about 10 years after my last read in such an archaic version of my native (and near exclusively used) tongue, I was unable to pursue. T'was like if I tried to read a spanish novel. Someday maybe I'll have the time to try again. My ego demands it.
Another example, easier to relate to and having more punch due to being on the english language and less time between writing and reading. I love Herman Melville. Read a translation of Mody Dick countless times and a while back I figured I'd try myself at reading "White Jacket" in the original version. I did manage it, but it was grueling even though a mere 150 aboutish years separated my reading from the writing.
Rare are those whom would have the willpower to try reading (a hypothetical) 31st century english.
Another comment while I'm at it. One must consider that through contact with other languages, words will thend to be use in the wring way, the semantics become twisted so as that a word loses its original meaning in the public mind. For example, english and french both use the word "administration" but the meaning is completely different in both languages. But constant exposure to the english language has made it so that the vast majority of french speaking people will use "administration" for its english meaning. On the top of my head I don't have a gallicism to point out to drive my point home but you get the idea.
So, even if syntax, grammar and all stay the same from the 21st to the 31st century, chances are words won't have the same meaning anymore.