Actually it hasn't been in development that long. The current version of TF2 has only been a year and a half.
And if you had read any previews or interviews in the last 6+ months, you'd know that there have been 3 (I think it was) internal versions of TF2 that were scrapped because they didn't find them fun. They haven't been working on TF2 for the last 8 yrs. Would you rather they release crap?
Which would imply that they've been working on the game for 8 years. They haven't been working on the same version we see now, but its still been the same game.