They are "stagnating now" but the long term problem isn't solved even with immigrants. So the point is that humanity itself needs to find a sustainable solution

They are "stagnating now" but the long term problem isn't solved even with immigrants. So the point is that humanity itself needs to find a sustainable solution. They definitely have the resources to deal with their problems, they just don't have the labour to utilize those resources. For example migrant labor is definitely a solution that Japan is working on.