The reason why authors published with traditional publishers are now only earning $8000 per year is also marketing. Traditional publishers do not spend on advertising these days.
So I'm not sure what your point is.
Yes, certainly there are some major bestsellers out there. If you look at who they are, with the exception of JK Rowling (who had a grant from the Scottish Arts council or something like that), they're all old writers.
They started up when the business model for book publishers was very different. That business model has not existed for 30 years, and each year, the budget for writers goes down. The commissioning editor no longer decides who gets published - the marketing department does. They look for people who already have a large following or who is some celebrity or politicians where sales can be guaranteed.
As for writers not having enough money to pay for advertising - of course. That was the point of going to a traditional publisher. However, as traditional publishers now ask you to foot the bill, and as they now earn less than indie publishers, I don't see your point.
And, yes, they compared the two groups as a whole.
https://www.allianceindependentauthors.org/facts/
Nine years ago, traditional authors earned £11000 per year. Now it's £8,000 per year.
QUOTE: YOU WILL HAVE TO MARKET YOUR BOOK. Many authors I work with want to traditionally publish so they don’t have to do any marketing. That’s not how book publishing works in this century. Even if you traditionally publish, you will have to do the lion’s share of marketing. This is why building an audience is important for every author.
