In something like Denmark's case, I could see an argument. In the US? Rich people pay nearly nothing, so I think forcing them to pay taxes is a slam dunk

I hate this argument.

If they're not paying their fair share of taxes then leaving is beneficial. It's like saying "yeah but if you ask the thieves to pay for their stolen goods they might leave the store"

I agree that immigration is a big part of fixing this.

I think if the population (replacement rate )declines too swiftly, you have a lot of old people and few young people. At some point it become exponentially harder to even keep your last generation's replacement rate, and you have spiraling population decline. See South Korea for the Speedrun.

If Japan steadily declines to 80M it could be fine. If Japan goes to 80M in a generation, that's very very bad for the chances of there being any Japanese people left in a few more generations