I personally think a good game,book, movie series should end with a grand finale. A huge boom and then a good or realistic end to the franchise, as the developer,wrier, producer or whatever, it is their job to grant their fans a meaningful and satisfying end to the series that said fan has invested so much time in. It must be given effort and pride in the ending, because the fans had spent all of their money in either buying the successive games, reading each book to understand what happens next in the story, or watching the movies to understand what will happen to their favorite characters.
As a maker of a series, it is their job to end it and their job to give the fans a good last ahow. However, dragging out a series and then giving the fans a lack luster or absolutely horrible ending would be a kick in the balls or a slap to the tits for the fans who enjoyed the series to said end.
That's just my thoughts on the matter at any rate.