Supernatural reached a point that dug into a very primordial, pre-biblical, existential issue. God vs Darkness, creation vs nothingness, yin and yang etc... One can't go on without the other. In addition, fans know supernatural a little to well a little too much and they just want to see their ending. It is very hard -impossible- to transfer the vague idea of creation, genesis, and "the beginning" on to the small screen. I believe they did a good job, it was a great season, not perfect, not a 10 but hey...:
"Endings are hard. Any chapped-a$$ monkey with a keyboard can poop out a beginning, but endings are impossible. You try to tie up every loose end, but you never can. The fans are always gonna b!tch. There's always gonna be holes. And since it's the ending, it's all supposed to add up to something. I'm telling you, they're a raging pain in the a$$."