When is a Trilogy NOT a Trilogy?

I loved the Millennium Trilogy by Stieg Larsson (The Girl with the Dragon Tattoo; The Girl who Played with Fire; The Girl who Kicked the Hornet's Nest).  Now apparently the trilogy will be continued.  Another Swedish writer, David Lagercrantz, has been hired to continue the series.  The Girl in the Spider's Web is scheduled for release in North America on Sept. 1, 2015.  The British newspaper, The Guardian, had an interesting article concerning this upcoming publication: http://www.theguardian.com/books/2015/jul/19/stieg-larsson-girl-in-the-spiders-web-david-lagercrantz. 

Will this sequel be as good as its precursors?  Should series even be continued by other writers after the death of an author?