The quality of fiction vs. the quality of non-fiction

Marcos Jazzan, a loyal MR reader, requests:

The quality of fiction seems to be decreasing relative to the quality of non-fiction, or am I just biased against active fiction writers vs. dead ones?

I agree with this assessment, and I see a few mechanisms at work:

1. A lot of good non-fiction is based on current affairs, which are always changing, or progress in science or social science, or biographies of previous uncovered subjects.  Fiction doesn't have a comparable source of new material, at least not since the modernist revolutions.

2. The internet makes it easier for people to be interested in a "culture of facts."  It doesn't help long narratives in the same manner.

3. For a given level of IQ, people are more likely to agree on what is a good non-fiction book than what is a good fiction book.  Internet reviews therefore make non-fiction purchases more reliable to a greater degree than they do for fiction.

4. Arguably literary fiction peaked in the 1920s, with Proust, Kafka, Joyce, Mann, and other important writers.  Could it be that fiction took a bruising from the rise of radio and film at that time?  Even if we compare the 1960s to today, fiction seemed to be more culturally central then.

What mechanisms am I missing?