The latter is linked to the described right below.
The author himself mentioned that this is a series where hard work pays off and it shows.
On the downside, due to Issei's fame in Underworld, he's too busy beating up supernatural invaders to the point of almost and potentially neglecting his wives and kids.
In order to reseal Trihexa and ensure that it will be defeated for good, most of the leaders of the various factions.
The low fertility rate became particularly problematic after the massive casualties from the Great War, which included the Biblical God and consequently, any increase in Pure Angel numbers.
The Leaders of recognize this and instead of crushing them, purposefully have them fight people too strong for them to kill but not strong enough to defeat them, so they can let all of their suppressed hatred out and move on.