People will continue to retire in their sixties and live longer with progressively more expensive medical treatment. Not exactly a road to riches, although riches aren't everything.
Germany by contrast has seen zero population growth over 40 years and did not have similar growth (or debt) problems of the sort that Japan created for itself. Germany's GDP has increased nominally by 750% over those 40 years (their economy also tripled in size in the 15 years before the Berlin Wall fell, with zero population growth).
If demographics mattered that much, Germany's economy would be - and would have been - a disaster like Japan's is.