Well it was said before that the cause of that is the gradual decline of the demons, and without an immediate threat living beings tends to become weaker and weaker.
See today's livestock: their ancestors managed to survive in the wild, but if you release them now, they will almost get wiped out.
It feels like there's a third party other than the kengan association and purgatory and it'll be a three way war...
or they'll be like the 100 compagny association of season one and backstab in the middle of the competition