Cool, Shiny, Schway
Some time ago I read a very interesting article on why the old people in Warner Brothers cartoons say things like "dagnabbit"1. (Edit: I have been helpfully pointed at the article in question by a denizen of Making Light; it was, unsurprisingly, on Language Log.)
The thesis of the article, as best I can recall, was based on the increasing urbanization of the US at the beginning of the 20th century. Around the time that radio was at the height of its power, movies were moving into being common, and television was just getting started, you could say one thing pretty surely about an old person: they'd grown up in the country. Of course this wasn't universally true, but in the 30s and 40s it was still vastly more common for the city to be somewhere you moved to as a young adult. So if you wanted to portray an old person, a good way to do it was to make them talk like a hick, like someone who'd grown up in the backwoods somewhere. And that accent mutated in connotation, from signifying "rural" to signifying "old".
But the fact of the matter is that the US is no longer a nation of farmers; it no longer makes sense to have an old person talk like an extra from The Beverly Hillbillies, because even someone well past retirement age is likely to have grown up in a town, if not a city. Add in the increasing influence of television, and you get the reason that regional accents are, if not precisely dying out, at least vastly less ubiquitous than they used to be; consider that I, who have lived all my life in a city that has a book2 dedicated to its odd accent, call sweet carbonated drinks "soda" instead of "pop", and have never heard anyone refer to Philly's football team as the Iggles unless they were exaggerating for comic effect. Tom and Ray Magliozzi sound like they're from Boston, sure, but they don't say "caa paak", and if you missed a few key episodes it's entirely possible to not know where Nick of CSI is supposed to be from.
It makes even less sense to have a character bust out the consarnits when the setting's in "the future". Time travel shows do this sometimes, where a character will be shown much older than normal and suddenly starts saying drat and ain't and fellers. It's silly and wrong.
Meanwhile you have things like Firefly, where the characters use mostly Chinese to curse in, but sound like stereotypical movie-frontiersmen for a reason; the whole point was to have a Western in space, and the writers used the accents to push that impression. Note that the characters from the Core Worlds (for which read "back East") say things like "reckon" a lot less. There were also a couple of amusing new bits of slang, like shiny for cool and sly for gay.
Oddly, the show I can think of that was set in the near future (Firefly was several hundred years on) that handled this best was Batman Beyond. In it, Bruce Wayne is elderly and there's a new kid, literally, in the bat suit; we get to see a lot of teenagers and their talk and slang was pretty authentic-sounding while still being little like contemporary teenage slang. The big substitution there was the completely invented "schway" in place of "cool". While I'm skeptical that a mere 50 years3 could be enough to dethrone cool--cool is to slang as Methuselah is to humans, in terms of staying power, and shows no signs of slowing--the fact that they didn't just attempt to spackle then-current slang with a few made-up words was, if you'll pardon the expression, awesome.
1: I can't find the article, alas, but I was reminded of it by a comment on, of course, Making Light.
2: Small and paperback, but a book nonetheless.
3: Beyond premiered in 1999, and the first season was set in 2040.