How much would Bernie 2016 cost you? (opens in new tab)

I don't really have an opinion on what the optimal capital gain tax rate is, but wrt your specific point of capital being mobile in the sense of being able to evade a higher tax rate (as opposed to being mobile in the sense of being able to choose what to invest in) - I'm not sure it is except for the few very, very rich people. If an American divests from US stocks and reinvests the capital in European or Asian stocks, AFAIK they still have to pay taxes for their capital gains, and Americans specifically even have to pay capital gain taxes if they move elsewhere, unless they renounce their citizenship. Certainly pension funds managing the capital of millions of Americans will pay a higher capital gain tax regardless of where they invest the capital and millions of Americans will not emigrate as a result. As to corporations, the US is making it tougher to escape its unusually high tax rate on profits by targeting "inversions" right now.

So while I'd guess that higher taxes will hurt growth at some point, I'm not sure that they're that easy to escape; perhaps I'm missing some obvious loopholes.

The US is not just a shining city on the hill; there are huge ghettoes and income disparity is sky-high in a way that is simply unimaginable to Europeans. Recently, these disparities have reached a point where both underclasses and lower-middle-classes feel they are being screwed a bit too much, because productivity gains from automation have gone entirely to capital owners. There is only so much you can tolerate in the name of "market adaptability"; after all, Europe might have its problems but from an economic perspective is still a powerhouse.