The decision not to have children is also not especially unique to those in economically precarious situations or those living in poverty. On the contrary, the demographic crisis affects everyone, including a well-off middle class that will have no trouble providing for its children. On top of that, while I support sound pro-natal policies, what we've found is that using financial incentives has very little to no effect on increasing birth rates.
Demographic decline reflects deeper problems in our cultures. We structure our lives in ways that do not respect optimal fertility, putting things off until "we're established in our careers". We define ourselves by what we own. We have a prejudice against large families, associating them with the shame of poverty, misery, backwardness, and a lack of education instead of the great wealth that they are. We have taken individualism to such an extreme that family and community life has taken a huge blow, and with it, the broad social support children would usually grow up with. Children have fewer siblings and fewer cousins to play with and grow up with. We have a dating culture that, instead of functioning as a way to find someone to marry to start a family, is recreational, aimless, and devoid of any desire for commitment. Expectations w.r.t. children that are financially costly lead to thinking that having more children would "deprive" them of a desired standard (this relates to keeping up with the Jones's). And what if you have more than 2 children in close succession? Well, because of safety regulations, you need to buy a larger car, because a basic sedan cannot hold three car seats.
Taken in aggregate, the cultural climate, as well as the attitudes it shapes, is not favorable.