I t hink they simply ran out of ideas. The main characters storylines are not as good as previous seasons, but I'm on disc 3 and they have introduced some characters I like a lot. So it breaks about even.
I just finished the last disc of the last season. I was very disappointed! I was expecting some drama for the final ending. When Don bought his motorcycle, I thought for sure he was going to have an accident or just ride off into the wild blue yander. Even though it was not the ending I was expecting, I'm glad I stuck with every episode so I can say I finished the whole series.