Important note before you all rage: this may or may not happen in our lifetimes (can't predict when, so it may or may not matter).
First and foremost, there are limits to growth. Seriously. There's only so much economic activity that can happen on Earth, the solar system, and even in the Milky Way. Growing at 4% per year after inflation is not sustainable forever. The Earth is not finite. Its resources are not finite. The main limitations are energy and non-renewable resources in general. Even with substitutions, we will end up using all of the solar energy that hits earth in a much shorter time than one might imagine (about 400 years). That brings with it some not good consequences (like raising surface temperature) but lets ignore that stuff for now.
So if energy is our limit in 400 or so years, why care about that now? Because growth has limits. It is physically impossible to grow at 4% forever. This begs the question, "how can one live off of interest/returns if there is fundamentally no growth?" How can we, as investors, make a living and retire early through a situation where the economy will not grow more?
If resources infinite, then non-material sources of economic activity (services, information, etc) will outpace the material goods until the material goods are cheap enough that anyone can afford them (due to automation, efficiency, etc). Since resources are, in fact, limited, then non-material sources will outpace material goods for a while until that limit is realized. At that point, the prices will rise steadily on the material goods as there is no longer a ready supply of them. This can be seen in some areas already (look at SF Bay real estate for one thing).
So, how do we invest with the end of growth in mind? This doesn't mean there's no growth. Companies will grow and collapse, but it'd be a zero-sum game overall. The only option I can think of is owning the means of production (owning a company, real estate, etc) which pays a significant enough dividend for owning to pay more than inflation.
In this scenario, ye olde index funde will rise in value with inflation and no more as the overall economy is not growing nor shrinking in any real amount.