The economists are worried because (retirement) savings are nothing more than a claim on future labor. As people grow older and are less capable of taking care of themselves other people need to care for them. People can be secure in their retirement when the number they have saved is sufficient to purchase the labor they need later in life.

But what will the price of that labor be when birth rates collapse and there aren't sufficient people around to do this labor? Retirement savings mean nothing when the people needed to do the work are not available. For wealthy western countries immigration is the only realistic solution address the shortage of working-age people. It's not a silver bullet, though. Immigration scares people and populist politicians will exploit this fear of change.

We should also remember that politicians in rich countries are very old. They care a great deal about anything that affects their own retirement. To young people who don't have high expectations for their quality of life in their last couple of decades on earth this panic about demographic change may seem silly. But baby boomers care and they are the ones making policy.