I didn't know Apatow said he imagined it going on forever, but as far as "original visions" go, that's probably wishful thinking and one rung down the ladder from David Lynch and Mark Frost planning on continuing to spin off Twin Peaks plotlines for years from the Laura Palmer plot without ever actually solving it.
I'm grateful that most of the cast went on to good careers; what's remarkable is that the biggest careers have gone to Seth Rogen and James Franco, who played the two least likable characters on the show. If the show had gone on past high school those two would have had to become more likable fast, which would be betraying the "get real" aspect of the show (a "get real" aspect would have had both of them moving on to god-awful jobs and going nowhere in life, which would have turned the show into a partial Roseanne), but if they're going to have Bill Haverchuck and Sam Weir making out with cheerleaders before the first season is up, that "get real" aspect must not have been very strong.
As for the other characters, I dunno--nerds in college? I'm guessing F&G would have made a good three-four season show, tops. Not something that should last 10 years.