Marketing sadlifer otakus with impregnating girls might not be the most fruitful strategy. Instead, in all those NTR/cheating donjons I read, it was always the black guys that boned the girls. And you know how in anime it's also always the rich european aristocrat family that moves into a mansion on top of the hill just outside the town? They're actually secretly inviting foreigners to come and fuck their women to solve the birth rate problem.
If what you say is true id have to put forward a parallel theory concerning the flood of NTR, forced and cheating tags.
These all being part of a larger long term strategy to destroy social and moral barriers that inhibit free sex.
With such barriers weakened they can ensure the future of their country through not only promoting impregnation with normal couplings like the OP theorized.
But also securing the less than willing portion of the population.
A best case example being.... an ugly mother fucker raping a lesbian
both of these minorities would normally not contribute to solving the population crisis for reasons like.
Being ugly- girls dont like ugly = virgin forever -> no help at all.
Being a lesbian- no dick = no babies -> no help at all.
Along with the addition of the ahegao tag which promotes the role of "victim"
i would say an upswing is inevitable.
It all makes sense now.