Smaller coins, sure. Its a legitimate concern.
But it becomes far harder as coins get bigger. Up to a local maxima of general global use and investment. Even harder as companies compete against one-another to do the same thing. What's stopping Chase Bank from buying up tons of coins and mining power to control Bitcoin? Well, every other human who owns Bitcoin, not to mention Bank of America and Wells Fargo.
Company interest in doing this will naturally correlate with usage. Which means, companies & governments simply wont do it until its too late and they can't. And if they did; if they jumped the shark; the coin's value would drop as users recognize this, and their investment would be ruined. Suicide-bombing is not in the self-interest of most corporations. The GDP of the US, the single richest cohesive entity on the planet, is still only ~25% the GDP of Planet Earth.
This sort of co-option of a currency won't happen, because a currency would either be so big that no entity could afford to do it, or a currency is so small that taking control would destroy its market and their investment.
