Supernatural has longevity in its genes. There's a reason it has outlived most other Sci fi/ fantasy shows that have come and gone during its duration. After watching other series either bow out with class, Penny Dreadful, or die a slow painful death, The Walking Dead, I would guess its strengh is the core characters. Sam and Dean have grown but remained the same people at heart. A car has become a main character viewers look forward to seeing every episode. Don't knock the dynamic duos, there hasn't been one this strong since Mulder and Scully.