Just heard about the study this morning. First, some generic, broad definitions of terms to guide discussion:Democracy
: "a system of government by the whole population or eligible participants"Republic
: "a state in which supreme power is held by the people and their elected representatives, and which has an elected or nominated president rather than a monarch"Oligarchy
: "a small group of rich, powerful people having control of a country's government"
With these definitions in mind, I would assert the United States is certainly not a democracy. The Founding Fathers definitively set up our government to not be a pure democracy but rather a constitutional democratic republic, having both democratic and republican aspects. The reforms of the Progressive era brought even more democratic ideas to the table, like the referendum and recall. But we're still a constitutional democratic republic, despite the ongoing violations of the Constitution.
While our government is still theoretically a republic, the study demonstrates that our country is drifting towards more oligarchic tendencies. Thus I would disagree with the study's main conclusion that the US is a bona fide oligarchy but would rather assert that we are a constitutional democratic republic with oligarchic tendencies. Why? Look at empirics. The ancient oligarchies of Greece had hardly any democratic principles but was truly rule of the few. Here in the US, the people can still vote out whoever they want, whenever they want, despite the fact that politics is now largely controlled by the big rollers who can essentially "purchase" a political position.
The study itself essentially admits that when it states
: "The central point that emerges from our research is that economic elites and organized groups representing business interests have substantial independent impacts on US government policy, while mass-based interest groups and average citizens have little or no independent influence." And from Business Insider
: "After sifting through nearly 1,800 U.S. policies enacted in that period and comparing them to the expressed preferences of average Americans (50th percentile of income), affluent Americans (90th percentile), and large special interests groups, researchers concluded that the U.S. is dominated by its economic elite."tl;dr version
The US remains a constitutional democractic republic with oligarchic tendencies. Although the US is seemingly becoming more about what rich people say and not what most voters say, the mechanisms for change still do exist, and the US has not become like ancient Greek oligarchies.