Yes, the US isn't a democracy. It's just a country that goes around the world demanding that everyone else be democratic. Even to the point that Bush Dubya, elected without a majority of the votes tried to depose of Hugo Chavez, elected with a majority of the votes, and the people who did the leg work, were supposed to be promoting democracy.
However that doesn't mean that in the modern time the US shouldn't become a democracy.
The reasons why it's not fit for the modern era would include:
The claim of the Electoral College is that it benefits the smaller states. The problem right now is that the EC only benefits twelve states. Literally presidential candidates only focus on 12 states. Wyoming, the smallest population, is NOT one of these states.
A Campaign Map, Morphed By Money : It's All Politics : NPR
If you look at maps 3 and 4 it shows advertising spending by presidential candidates.
You have 41st state in terms of population, New Hampshire, then 32nd state, then 31st state, then 22nd state, 21st state, 20th state, 12th state, 10th state, 9th state, 7th state 5th state and 3rd state.
Hardly representative even of the smallest states. Only THREE of the 12 states are in the bottom 50% in terms of population size. Meaning 22 of the smallest states are almost completely ignored.
You have the reality that people now vote for a Federal President. Yes, the whole "states have the power and the federal govt represents those states" doesn't exist. The federal govt has supremacy, and yet the election kind of does a middle ground of promoting some states and not others.
The House elections are probably even worse. Gerrymandering is so bad it often means people don't get a say in who their representatives are.
FPTP is a bad system because people ended up voting negatively. This can be proven with the German elections where they have FPTP and PR on the same day. In 2017 8% of people changed their vote from big parties with FPTP to smaller parties with PR. And this is with them already knowing that PR will decide the outcome of government. It was 10% in the election before this.
So people don't vote FOR Republican or Democrat in large numbers. They vote AGAINST Republican or Democrat in large numbers. You're looking at about 33% to 50% of people who do this.
The Supreme Court is ridiculous. So, the Republicans have won ONE popular vote since the 1980s, and that was Dubya's second election. And yet the Republicans have had 50% of Supreme Court justice picks and more Federal court picks. This has become a highly partisan issue and the fights are ridiculous. The Supreme Court is not representative of the country and yet has so much power. It was not designed to be so partisan. Nor was the presidency or Congress.