I predict instead the democrat party will split into two new parties: the Moderates and the Leftists. The Moderates will basically be the democrats as they are today and the Leftists will consist of politicians that hold more radical Leftists views.

yeah this is basically always how it was going to go down, essentially also what happened to the republican party post civil war. yes there continued to be a democratic party but the democratic and republican parties of the late 1800s and beyond both borrowed heavily from lincoln's GOP. the democratic party of the confederacy largely died out even though the name and many ideas continued on.

i would imagine a similar thing would happen here where the republican party won't actually disappear, but will take a more moderate approach while the democrats go further left. in 40 years we are likely to see republicans where democrats are today and democrats square in the actual left.