Birth is not the limiter, childrearing is. Synthetic wombs are more expensive than just having surrogate mothers. For the same reason that synthetic food is more expensive than bread and cabbage.

The actual counter to fertility decline, may be AI teachers. AI will radically close the education gap between rich and poor, and lower the costs. All you need is a physical human to supervise the kid, the AI will do the rest, from entertainment, to education, to identifying when the child is hungry/sleepy/potty, and relaying that info for the human to act on.