I would disagree on two points; that a cylinder is desirable and that you need immigration to fix the inverted pyramid. On point two; the inverted pyramid is I

I would disagree on two points; that a cylinder is desirable and that you need immigration to fix the inverted pyramid. On point two; the inverted pyramid is IMO largely a symptom of a changing demographic layout. Younger people create less families and briefly cause the pyramid to invert while the ripple of low births goes through the entire lifetime of a human. On point one; if your pyramid is a cylinder, it implies people die at any age in a uniform distribution. It is unlikely this will be the case ever. IMO a somewhat slopped pyramid is okay, people tend to die as they get older and this lessens the costs on social welfare systems (plus being more realistic).