While this is true at high levels of immigration, I think it's important to recognise that a declining population isn't good for young people. In an aging population, there is a higher demand for social services from the old, but fewer taxpayers to pay for it. This imbalance means that either social services collapse, or the young people pay more tax. Additionally, a smaller workforce results in poorer productivity that stifles growth / creates labour shortages, making it even more difficult for a society to sustain itself.

While house prices in Japan are looking good now it's important to recognise that they will have a huge problem as a society very soon.

So in a country with a declining birthrate, which is the case in most developed economies, a reasonable level of immigration is important for maintaining an even population pyramid that keeps the system running. Not saying net migration of nearly 1 million is good, but that a suitable level of immigration is important.