This was said even by John Carmack (trying to find the youtube video, it was Id's weakest selling post Doom game) but quake 3 wasn't really ever that big. It didn't crack a million copies (the older games sold much better, Quake 2 outsold all the other Id games until Doom 3, confirmed by Tim Willits here at 40:40 ) and regardless what many think of post quake 3 Id, Doom 3 was and still is the best selling Id game at over 3.5 million copies, as stated here:

Just wondering, but why do you think Quake 3 was kind of a flop compared to previous Id games?