Did they ever stop? I don't think I've ever seen liberals (party or base) grapple with how sinister that primary was.

It's because it is not an especially significant incident within the framework of the system. Parties are private entities with a long history of shenanigans going back to the very beginning. The parties are actually a lot more "democratic" than they have been at other points in history and the 2016 shenanigans were relatively minor. If Bernie were making a stink about it things would likely be different but he isn't so only Republicans and their allies still care. What actually matters is state power, specifically how it is organized and wielded has changed in increasingly anti-democratic ways. Liberals are definitely starting to wake up to the reality facing the US, you can see them freaking out with some regularity in various opinion columns over the past several months. The situation hasn't fully sunk in yet with all of them is all it is, there are many who are still in varying degrees of denial. There's also a huge number who are somewhat naive, barely pay attention to politics, and are largely ignorant of reality until it becomes unavoidable. As reality becomes increasingly unavoidable we will see more and more of these liberals waking up and getting radicalized. They seem to historically be slow to recognize what is happening and have to be dragged into actually fighting fascism but they should get there eventually.

Ross Perot would like to have a word with you, lol