The US demographic wave hasn't stopped; that's why, from at least a European long-term investor perspective, the US is still a good bet compared to countries that have already peaked: Italy, Japan etc.

There are many paths to wealth. Germany is rich because of - in part - the almost total rebuilding of a ruined nation and the creation of a pretty effective post-war consensus in West Germany. In fact, I'd argue that an important part of the US's success were also an effective post-war consensus (different, but also hugely effective).