There is no “zero” immigration policy, this is a strawman. Controlled immigration is different from opening the floodgates. Also weird to admit that no country has reversed its birth rate problem, but still insist upon massive immigration being the solution.
In the united states the birthrate problem is largely climate change panic (which is not a reason to not have kids because other populations / immigrants start accelerating their birth rates), and then because of heavy migration and other market factors, home prices have skyrocketed. Many couples don't want to have kids until they at least buy their "starter house". I'm not very familiar with Japan's problems, but I…
Their work culture actually rivals the US for toxicity.
And they dove into the modern technological society fully before anyone other than maybe Korea... Who is also having birth rate problems