You have fair points, but I disagree with some conclusions. Demographic change for example hits everyone at some point. Concluding that it will lead to a competitive disadvantage specifically for Germany doesn't seem plausible to me, at least not in the long run. In the near future it might be, the question is how well we'll adapt. Japan for example is struggling with demographics as well and they still do OK, albeit not amazing.

Moreover capital doesn't necessarily decline becaue of a demography shift. State funding and buying power in Germany might, but the state is trying to reduce its debt, which opens up opportunities for future investments. Fewer people also mean that (hopefully) fewer investments into certain things are necessary. And as you mentioned, businesses are exporting and expanding into other countries. This means they extract capital there which could be directed towards Germany. You "just" need to capture it by making investments atteactive. I think the R&D area is a possible lifeline here. That said, private capital is a completely different story to begin with anyway. It can be used for investments that drive dividends toward Germany. There is no need for that capital to decline at all. It could of course, but I don't think demographic change will be at fault if it does. Capital tends to attract more capital.

Finally I highly question the need for military power in order to conduct global trade. The US aren't an economic superpower because their ships are escorted by destroyers, because pirates are afraid of retaliation or because, in contrast to other countries, they aren't robbed of their goods. It might have opened some doors and it makes it hard to blockade them, but that's assuming people want to blockade you to begin with. And Germany doesn't stand alone. With a strong EU and growing military spending, I'm actually hoping for more independence in every regard, including military, in the future. The cold war is over and the Soviet Union is no more. It's time to take care of ourselves in Europe and make our own decisions.