With Terry not writing any more Discworld books and the next book being 'snuff' of it, I don't believe it. Stephen King has said that he wasn't going to write any more Dark Tower books after the last one, now he IS writing a new one that takes place between book 4 and 5.
As he and many other authors have stated that when a book needs be written it will right itself.
Even though he is going to write another series, he could alternate years of release. So lets say that 2012 is Snuff, then 2013 will be the new seres and 2014 will be DW, and so forth.