Populations aren't as simple as: this side just keeps increasing forever.
There are multiple forces at play that stabilizes things. You don't get to say "you don't understand statistics" because you limit yourself at a too basic level. From your understanding of population dynamics, the western world should have kept increasing its population, but reality has shown us that it not only stagnated, but it's actively decreasing in size.
Populations tend to stabilize themselves at healthy levels.