On the subject of declining advances from trad publishers, many people have commented that it's counter-intuitive when authors can self-publish. Surely if SP authors have more options, advances ought to be higher to attract the best? But that hasn't happened. My theory is that a few SP authors have done spectacularly well and have been picked off by trad publishing. That's where all the telephone number advances have gone, to the few top SP sellers who can guarantee sales. Those who go through the slush pile are deemed to be mid-list authors, with modest advances to match. And as long as authors accept those modest advances, they will never rise.
As part of the business model, an advance going to an established sure thing almost doesn't count. If it's not a risk, then it's outside of the VC model. It's almost like a side business. Publishers invest in a lot of books so that the winners can fund the losers. They don't make much if any of a profit on most books. They see most of their books as a package deal, and the sure thing by a celebrity or established self-publisher isn't part of that package.
If you isolate books that are part of the VC model, advances have gone down. The only reason I can see for that is that the sales potential of any given new author has declined. That is, it's harder for a new author to gain traction than it used to be, even after they're traditionally published. Self-publishing has let a lot of new authors into the market. It's helped a lot of new authors to succeed. But it's also made it harder for a good book to gain traction by drowning them out with slush. It's a flaw in the model that needs to be resolved.