Is the US losing its dominance in science? A few points:

• A relative decline is inevitable.
  
• European science was dominant before 1950, so its resurgence across the Atlantic is just a reversion to type.
  
• Since ~50% of humans are Asian, and the prominence of Asian civilization, especially China, is a persistent fact of human history, a larger contribution to science from that region is also not surprising as economic parity is achieved.
  
• But: I am skeptical that Asian culture is as conducive to risky heterodox thinkers as American culture. A lesser process might occur in Europe, and the demands of the welfare state might in the long term siphon money away from basic research throughout the EU.
  

So, I suspect that America will still be in a class above its weight because of a steady migration of innovative thinkers who want more freedom to take risks.