Now that the Winchesters know that demons can be “cured” and since they were once human souls, my prediction is that it would not be morally justifiable for them to close the gates of Hell, since they now know they can save them all instead of shutting them in so the demons will be left to gnaw on each other and the humans in hell for all eternity.

In the same manner, Castiel sealing the gates if Heaven would grant all the angels to conduct their wars to completion without the distraction of them being responsible for the humans’ future. Thus sealing Heaven is another bad idea born of a mind that thinks locking up a bunch of people who want each other to die would get them to calm down.