Why is it even a problem if fertility falls much faster in rest of the world and US also has massive immigration unlike almost all other countries? American imm

Why is it even a problem if fertility falls much faster in rest of the world and US also has massive immigration unlike almost all other countries? American immigration is also almost entirely Christian and whitest of immigration to any other large country, so it's not even making America genetically worse. It's only a one-generational integration problem, which always existed and America knows how to handle it very well, it's been there since inception. And well, majority of fertility decline is a consequence of successful handling of a major social ill - teenage pregnancies. People stopped having entirely unwanted kids at the time in life when they couldn't handle it. And yes, a result of this (but not the cause!) is the massive increase of parenting time/workload - not because past generations thought that letting their kids fend for themselves was a good idea, but because they were fucking teens who tried to figure their lives at least minimally with unwanted kids on their backs! Those kids were constantly beaten up, neglected, and grew up drug addicts who stole stereos from cars and dealed crack in 1980s. Thank God that kind of fertility is gone.