The latest post-2008 hierarchy escalation stage is just a few years old. The austerity effects will take another decade to play out and affect 2100 projections.

Fertility of the poor will likely meet the conditions (and demographic dynamics) of the 19th century. Their resource limitations is already a model of a wide civilization collapse. Before long, we will see how much poor population can be supported in this economic regime.

As for the middle class mysteriously lower fertility - the futility of their race for sufficient wealth and status is getting yet more obvious. You either hit abundance of a lucky rentier-investor and typically have a proud set of kids; this applies to many successful conventional professionals "earning" significant portfolios as an extra. Or you chase elusive fortune with a demanding work, mindfully continuing to compromise your biological chances. The financial food hierarchy is not explicit, but its reproductive effects (on primates) are increasingly just as in a jungle.