Why does WGS84 refer to so many different things?

Suppose I decided my house was the centre of everything and I wanted to define a coordinate system based on my house. I'd call this the Baz2017 Geodetic CRS. Now I need to define the Prime Meridian for Baz2017, and that will be the line going through the exact centre of my front door. That is now the Baz2017 Prime Meridian. Next I'll measure the shape of the earth... okay, its 1276 furlongs in radius (I may be way off here, but the Baz2017 Geodetic Consortium is not known for its accuracy) and the earth is clearly banana-shaped with various parameters, so that defines the Baz2017 Ellipsoid (which is really a Bananaoid, but no matter).

The Baz2017 CRS is valid for the whole world, and there's an EPSG code for that, Area of use EPSG 1262, so we'll use that. If Baz2017 coordinates were only valid for a range of 200m from my house, I'd have to define a new Area-of-use, the Baz2017 Area-of-use, and try and get an EPSG code for that.

So here you can see I've used "Baz2017" several times to refer to different components of the coordinate system, in the same way that WGS84 is used formally in your diagram. Informally, I can say "My address is (0,0) in Baz2017 coordinates" and I'm referring to the CRS as a whole. If I want to refer to the ellipsoid or other components, I'd probably be more explicit.

Do not use Baz2017 coordinates for navigation.

So what's the most common usage of the term WGS 84 - the ellipsoid, the datum, or the CRS?

They are really the same thing. Roughly speaking, WGS 84 (World Geodetic System 1984) is first of all a datum, which in GIS world means a set of parameters and a model defining the earth's shape.

The specific model used in WGS 84 assumes that the earth's shape is an ellipsoid (which is a close-enough approximation and a commonly used assumption). That's why people talk about the WGS84 ellipsoid. By comparison, there are also models that assume that the earth is a globe (e.g. Google's Web Mercator?? at some point of time).

When you represent locations on the ellipsoid directly using angular measurements such as latitude and longitude, the ellipsoid itself establishes a (ellipsoidal) coordinate system. If you use that directly as a coordinate reference system instead of projecting it onto a flat surface, this is called a geographic coordinate system (GCS), as compared to a projected coordinate system (PCS). This is why WGS84 is also a CRS.