I think it's up to the young to take care of the elderly and perhaps their welfare state is not as generous as ours. I don't know enough about Japan so you'll have to tell me the facts but we need to reform our social security and medicare/medicaid if we are going to cut off immigration. But we know that's not going to happen.
We have almost 60 million people on welfare in the U.S., the vast majority of them able bodied. We pay them to stay home and do nothing, except vote for democrats, that's what they're really getting paid to do. There is your pool of unskilled labor but we can't access it because they're not going to work if they can be paid to do nothing. The argument that we need immigration is bunk, we don't need more unskilled workers, we need to stop paying our own not to work. Even Obama acknowledged the problem by saying "there are some jobs Americans don't want to do". My answer is who cares if they want to work, starvation is a powerful motivator. Stop feeding them and they'll work.
I agree it won't happen though, too many people are getting rich off the destruction of our country.