What suggests a stable population to you? Developed countries are all dropping below replacement fertility rate, sustained only by immigration and the higher f

What suggests a stable population to you? Developed countries are all dropping below replacement fertility rate, sustained only by immigration and the higher fertility rate associated with recent immigrants. I haven't seen any evidence for a stable point of population - it looks like it will grow, until it begins to decline in the not-too-distant future. Then does it ever stop declining? Does it stop declining at a point we can sustain a technological society, much less technological progress? That's my vote for resolution of the Fermi paradox.