Imagine Switzerland bordered Russia... would they spend more or less on national defence? Does that mean that the EU is subsidising the defence budget of the country with the highest average wealth?
I agree with you that the economies of most EU countries cannot sustain US military spending levels.
My theory is that since the rich people in those EU countries are amassing wealth at a faster level than the economy is growing then we should use some of that growing wealth to ensure that the economy (and country) are properly defended. Also use the funds for education and infrastructure.
It turns out the US can learn from history and other countries when they see a model that suits them.