As for the question of quality, I read about 50/50 indie books vs. trad books nowadays, and it seems like the bell curve of quality is about the same for both types of books.
However, I can say for sure that the two worst books that I’ve read in the past year, which were so bad that they stayed with me and I must restrain myself from ranting at the crappiness of them, were both traditionally published. My book club ranted at both these books last night, so it’s not just me. These were actually outliers on the quality curve, a two-book blip on the left end of the long tail, they were so bad.
I have not read any indie book that approaches the sheer badness of those two major NY-pubbed books.
It seems like when an indie writer is bad, they’re amateurish in safe, predictable ways, and sometimes you can actually see them improve during the book.
When a NY-pubbed book is bad, the writer is arrogant and blind to how terribly the book has gone wrong, because they are a Professional NY-Published Writer, Dammit, and then the book gets worse as they go farther astray and defend their terrible, terrible choices. The badness becomes exponential.
Yes, quality is not the exclusive domain of the large publishers, and shlock is certainly not limited to indies.