Lucky then, that immigrants don't age. Otherwise you'd be turning your country into one hell of a demographic Ponzi Scheme...
So high immigration now is just kicking the ball down the road while destroying your local culture and people in the process.
But I don't really agree this is the appropriate equivalent at least for the "spirit" of a Ponzi scheme. A pyramid scheme exists only when a pyramid is disguised as something else. When it's designed and presented as an actual pyramid it's not a scheme. In the design of the pension system there's no intention of fraud involved (only lack of education maybe), and the system is constantly and very transparently adjusted to widen the pyramid base and narrow the top (increase retirement age, contributor base, or individual contribution).
And yes, the whole economy and benefit systems, including pensions, assume this. This is why it is difficult to face the fact that population cannot keep growing, and in fact it s forecast that will stop growing at global scale by the end of the century. It is even more unsustainable combined with the objective of solving the climate and environmental crisis.
There is a dishonest incoherency with our governments, and just incoherency with large sections of the public.