It is a good article, though it kind of misses the forest. Primaries are just how political parties choose a nominee. They are democratic to the extent that there is an election and the winner is the nominee. But because parties limit the options, you don't have a very diverse selection of candidates. Bernie wasn't popular because voters are wacky - Bernie was popular because a wide swathe of voters likes what he was selling. But because our elections end up having to be a Republican v. a Democrat, instead of several strong candidates, we get two.
Well, the article might have missed the forest, but you may have missed the trees.
The point is that the parties have lost their ability to pick reasonable, electable candidates for whom the parties can reasonably be held responsible. The bad old days of smoke-filled rooms and handshakes and deals may have been a better system than letting the most angry and energized fragment of the party membership pick the candidates.
In general, I think we should be very modest when we talk about making big changes in a very complex system that has more or less served us well for 230+ years. We should understand very thoroughly why something was designed the way it was before we start thinking about eliminating it or radically changing it.
I think the 17th Amendment changing the election of U.S. Senators is an example of failing to be careful. It fundamentally changed the relationship between the House and the Senate and between the states and the federal government. And not for the best, IMO.
Was it intended to do that? I don't know for sure, but those Progressives were pretty clever, and they wanted more federal power at the expense of the states, so I know how I would bet if my life depended on it.