I agree that Japan has its own problems. No solutions are particularly good if they can't get their birth rates up. Singapore also has low birth rates. What problems are preventing high-IQ people from reproducing might be something that EAs should look into.

"How much immigration to allow" and "precisely what kind of people should we allow in" can be related, because the more immigration you allow, the less selective you are probably being, unless you have a long line of qualified applicants. Skepticism of open borders doesn't require being against immigration in general.

As you say, a filtered immigration population could be very valuable. For example, you could have "open borders" for educated professionals from low-crime, low-corruption areas countries with compatible value systems and who are encouraged to assimilate. I'm pretty sure this isn't what most open borders advocates mean by "open borders," though.

The left doesn't "want" a responsible immigration policy either. For their political goals, they want a large and dissatisfied voting block. And for their signaling goals, it's much more holy to invite poor, unskilled people rather than skilled professionals who want to assimilate.