It's not plummeting because it's always been like this for years, although the addition of adventures helped out a little bit with the general quality of levels on the workshop.

Yeah but like a couple months ago the most popular tab below most popular over the week was like a bunch of actually pretty awesome levels like The Silly Space Escapade to name one. Now there is... free skillpoint. Wow.

The overall quality has never been good. The issue is that most of the creators who made good levels have quit or drastically slowed down level production. With less good levels to weed out the bad ones, the workshop is more saturated with low quality garbage and spam.