Editors are incredibly important! There are a lot of substandard pieces of writing out there, from people who don't see the need for an editor.
I have to say that I earned the most money from a book I self-published some years ago. However, it was written to fill a need in a niche market, so it isn't surprising it did fairly well for a self-published book. When I published a book with a big publisher, I found out they consider selling 5,000 copies to be successful. That's a long way from selling a million or so copies!
I think it's wonderful that the internet and print-on-demand have made it possible for anyone to publish a book. However, if you aren't already known in your field, or you don't have a pretty awesome marketing budget, you aren't likely to sell a lot of books. In fact, one of the things publishers look for now is writers who already have an online following so there's a ready market for their books.
I find the world of writing and publishing to be fascinating.