The next issue of 10%.
the reason this works are
1. inflation may run at 2-4% LT and cos pass these prices. Nominal stock prices reflect this
2 Index returns reflect pop. Growth of 1-2%, productivity growth of 1-3%.
3. index returns also reflect more efficient companies that grow faster than GDP (M&A, mgmt incentives etc)
add it all up, you easily get 10% or more.