He doesn’t know that Metatron is free.
He doesn’t know that a) Rowena did the spell to free him.
b) Rowena has the book of the damned.
He doesn’t know that Sam tried to Kill Crowley.
In short, he thinks they only have the darkness to deal with next season. he doesn’t know all the villains are out and roaming the world!
they may not have time for ‘monster of the week’ next season.