People speak the way they speak. If language changed like your totally unsupported bullshit statement claims, then people would lose the accents they were born with, which for the most part they do not. My grandmother would have stopped calling her couch a "Davenport" if that were true. It isn't. It's totally unsupported by any science or anything approaching science. It's a retarded comment you pulled out of your ass, like everything else you say.
Yeah, because your grandma is scientific evidence. Bravo.
I'm sure you haven't met many recently, given your utterly stupid views on them and all, but immigrants change their accents. And The Elves were immigrants. Going to entirely new places. Sure, they didn't have people to meet and talk to in those places, but they were seeing things entirely out of their experience.
There's a reason the OED adds new words. And why grammatical constructs change all the time. Is it faster due to the internet? Sure. But Elvish language doesn't change fast, which is the point.