Do we? When was the last time a country decreased its Gini coefficient?
Conversely, we can observe the effects of policy since the 1970s and declare it to be a failure. Despite long periods of economic growth, income inequality continues to grow.
Also IIRC the US improved it's gini coefficient for the one year 2009? 2008?; a lot of rich people suddenly got less rich until they got bailed out.