What???? Why would we have to do that? Runaway depopulation is the biggest issue facing the world? We have billions of people on the planet. GPD not going up is NOT an existential threat.
Also, unless you throw the elderly to the wolves, having very low birth rates leads to a huge drag from having too many retirees to support with your shrinking working age population.
Not to mention automation is only increasing. Japan will have a booming robot export business going in a few decades.
A lot of these arguments about "needing" immigration to support aging populations are just thinly-veled neoliberal excuses.