Why does it look like this? This appears to be affecting both Eevee and Cycles in similar ways, although I understand the shader view is Eevee only. My Cycles previews are also very "bright" or desaturated. It is very hard to get any darkness into them. The HDRIs themselves look desaturated in the background.
If you set up this same shader it'll appear darker for you.
When I open a new project it goes back to the fully opaque look and my HDRIs are also fixed. I can obviously give up trying to fix this and just start a new project and import the assets but I prefer to know why this happened so I can fix it.