I do not really understand what you mean with "The rest is inevitable", for example I do not know what exactly you mean with "the rest".
Perhaps I over stated the case by saying inevitable. It's quite feasible that the consequences of highly fertile people overbreeding will be a reduction in civilizational complexity, which will lowerthe total population.
> If you accept the heritability for age at first birth has not been measured wrong, then the hypothesis put forth by this paper will come about.
First you argue that if the paper is right, the measurements must be right. Then you argue that if the measurements are right, the paper must be right. Both arguments seem illogical, especially when used in that combination.