> the return on investment has been slowly and constantly going down since at least the 70s

Curious if you have a source for that?

> What does "adjusted for reinvested dividends" mean?

Dividends are payed out in cash and this means the analysis assumes they're used to buy more shares.

There's an argument that the American economy in the 20th century is itself a selection bias. It's the most successful economic period in history. Other countries in the same period or the world economy in other periods do not have such spectacular returns.