it says that season 6 could be their last one.
According to this article Fellowes states that
ITV commission each series on a year-by-year basis.
So they don't know yet if there will be another season or not. However, he goes on about a new show he is going to write for NBC and that he doesn't want to jiggle with two shows at the same time. That's why season six could be the last one.
I believe that having six seasons instead of just five won't hurt the quality of the show.