while I do think that publishers take more than their fair share, I also think that they do provide quite a bit of benefit to the industry, since they serve to ferret out the diamonds in the rough
You keep saying this, but I would say about half of the ' properly published' books I have bought over the years have been junk. And this is in the field of factual books ( I don't bother with fiction ). That's no better than my record on buying self-published books.
And it's actually self-published books, or those from tiny speciality publishers, which are considered by the market to be most valuable years after publication.
Mainstream published books? Just rehashes of what someone else wrote and once the marketing-induced fad dies off in a year or so they'll be worth pennies on the second hand market.