Twilight Imperium xpacs turn a long game into a entire weekend. Rule upon rules. These xpacs are exactly why the player base for this game is extremely small. Or more likly people only play it a few times.
Its quite the oppossite actually. The vanilla game is without questions the longest version you can play.
This is the Forbidden Stars forum so I don't want to get into too many specifics, but most of the X-packs have created more fluidity and routes to victory points shortening the game pretty dramatically. I agree its a niche game and has a small audiance, but that is not because of the expansion packs its because its a long epic game to begin with and traditionally in board games a game that takes more than a couple of hours to play automatically has a much smaller audiance. Forbidden Stars isn't going to be a mass market hit either for that very same reason.