They actually have a somewhat open door policy for the weebs through JET. If your an American with a college degree than your welcome in Japan. There just aren't that many of them and those people also don't tend to have children.
There are a ton of people in the Philippines and S.E.A that would do anything to get to Japan even if the work is hard because the standard of living is so much better, much like Mexicans and central Americans wanting to come to the u.s. They would also be more willing to start families. Like the u.s. though they don't like brown people and only let a limited amount of them in legally.
This is just like Canada banning foreign investment in real estate. It admits there's a problem, data harvesting , homes as investments, but just solves a small part of the problem pertaining to "foreign bad guys" while ignoring the larger domestic issue.
All it does is make the government look like they did something without actually confronting the powerful interests that are causing the problem.