Your reasoning is just historically uninformed https://en.wikipedia.org/wiki/List_of_countries_by_populatio.... There are at least a few generations of lag time between a population boom and economic effects, but Europe's former population size (and density) was the key to its current prosperity.

France would likely be more populous and therefore richer than Germany, had it not secularized earlier and thereby reduced its fertility rate. With all of Europe being highly secular and extremely low fertility, it will (sadly) lose much of its prosperity and cultural significance. Negative TFR societies are simply not sustainable - they will by definition collapse unless they aggressively revert the trend.

See https://ourworldindata.org/grapher/children-born-per-woman?t...