Considering Fable II has sold nearly 3 million copies - damn good for a console exclusive - and if Fable III gets even a million sold, it'll still probably have a sequel. So, last game in the series? Doubter.
so long as the cash keeps rollin in pm will make games, irrespective of the quality of those games.
i think aside from final fantasy, fable is the one game title that sales numbers do not affect the qualty of the game.
however- unlike ff, due to the fun and overally light hearted nature of the fable (well fable 2) games, you can enjoy the game immensely even with glitches and poor story or generic gameplay and generic combat.