They can't run this show forever... Spoiler: the Season Six Cliffhanger Deb just saw Dexter serial-kill someone!!! Is she just gonna keep quiet about that for three more season?!
I think season 4 was the peak, season 5 & 6 while getting very good ratings slipped down a bit, though don't hold me to that
In the books, that happened at the end of the very first one. Considering that the series has been shifting towards more and more procedural elements, I suspect they'll just have the two characters working together before long.