The world is flush capital which is driving down returns. We don't need to try and increase it as people with a surplus are always going to invest it.
I agree there's a problem there: we seem to have more capital than we need, yet we also have less savings (in the sense of people deferring consumption) than individuals need to have a comfortable retirement, for example.
I have yet to see good solutions for this problem.
In other words, if you just hold some money for a few years and it doesn't lose value (interest rate equals inflation), you _already_ owe tax on it. That's just broken.
If we did capital gains in inflation-adjusted dollars, the situation would be somewhat different.