In my experience, trade authors who are earning a decent living on advances and royalties still aren't ecstatic about their relationship with the publisher. Quoting from the correspondence of Frederick Marryat, a successful novelist writing in the 1830's and 1840's:
Your remark as to the money I have received may sound well, mentioned as an isolated fact.; but how does it sound when it is put in juxtaposition with the sums you have received? I, who have found everything, receiving a pittance, while you, who have found nothing but the shop to sell in, receiving such a lion's share. I assert again it is slavery. I am Sinbad the sailor and you are the old man of the mountain, clinging on my back, and you must not be surprised at my wishing to throw you off the first convenient opportunity.
The fact is, you have the vice of old age very strong upon you, and you are blinded by it; but put the question to your sons, and ask them whether they consider the present agreement fair. Let them arrange with me, and do you go and read your Bible. We all have our ideas of Paradise, and if other authors think like me, the most pleasurable portion of anticipated bliss is that there will be no publishers there. That idea often supports me after an interview with one of your fraternity.
Most of the "successful" trade authors I know, successful because they are always in demand to write another book for a modest advance, don't actually earn their primary income as authors. Many hold full time jobs in the professions or make their living as speakers and consultants, and some have sufficient wealth or retirement income to render any royalty payments a pleasant bonus. On the other hand, the common definition of a successful self publisher is one who makes a living at it. Therefore, successful self publishers earn a higher median income than successful trade authors by definition. I won't say I higher average income, because it doesn't take many J.K Rowlings or Rick Warrens to raise the average.
I think most unpublished writers would argue that anybody who gets paid to write a book is a successful author, without regard to what happens down the road. I remember very distinctly being out running one day in the mid-1990's and swearing to myself that if I could just break into the trades and earn $20K a year as an author I'd be happy for life. Not long afterwards I started earning that income and more as a trade author, but my attitude changed by the third book, despite the fact it brought in well over $50,000. As I learned more about the business of publishing, I went from feeling lucky to feeling I was trapped in a bad contract. An agreement first signed in 1998 continues to tie my hands in an area where I once had a terrific Internet following, lost to the publisher's inflexibility on managing the intellectual rights.
Turning down a trade contract to self publish is an iffy proposition for most authors, and I generally discourage anybody writing fiction from doing so. For nonfiction writers, the financial results are going to vary on a case by case basis, but if you care about control, self publishing will always be the better choice. Control isn't just about the editorial process, when to release a new edition, and whether or how to publish on the Internet. It comes down to the most fundamental question of all facing any author - What's my next book?