There is still plenty of source material ahead. As I mentioned upthread they are only at the point the comics were when the show started. There are 75 issues of story material, and counting, left to tell.
I doubt it will last that long. Eventually, Lincoln, Reedus, McBride and others will want to leave the series, and without those anchors, TWD would have a hard time trying to maintain strong protagonists, as the 2nd tier actors/characters might be interesting (after several seasons), but some are nowhere near as developed as Rick, Carol or Daryl.
This season has moved very well so far, but I just hope that with every new plot taken from the comics, it does not just transform into more of the same meet new enemy/suffer/group member death(s)/enemy defeated / move on template they've used since season 3.