How about we define success as "able to make a viable living off of it."
Then, I think you need to go to print.
Folks on EN World have, imho, an overly optimistic view of the pdf market. Yes, lots of folks here, on EN World, like pdfs. However, I have seen no credible evidence that the bulk of RPG players make use of them. I think the market for them is much smaller than many might expect.
Back in the day, that generally meant that you had to be a big name like Shadowrun, because printing and distribution costs cut into the percentage of the sale price that ended up back in the hands of the authors. Today, a much larger percentage of the sale price of a PDF (essentially all of it) goes into the pockets of the authors...
Folks have done breakdowns of the costs of producing works - while you get rid of the cost of printing and most of the distribution costs, your artists, editors, and writers still need to be paid. So, it isn't 100% profit, by any means.
In addition, have you noticed that folks tend to gripe when you try to sell a pdf for as much as you'd ask for a print copy? It seems to me only WotC has the clout to ask that - so while you aren't paying printing and distribution, you also aren't able to ask as much per copy.
I am not sure that this comes out positively for the pdf publishers.
In addition, pdfs have a major marketing disadvantage when compared to print. For a print work, people can see it on a shelf in a store or a friend's house. They can take it off the shelf and browse through it, or borrow it from the friend. The physical object helps get the product into people's minds.
Meanwhile, for a pdf, the people really have to come looking for it before they ever see it. If I am not specifically searching for your pdf product, I am unlikely to ever encounter it. It is may be sitting on my friend's hard drive, but I don't peruse his electronic library. I don't see the thing on the gaming table. I don't see it on the shelf when walking through a book store. Banner ads on websites are cheap, but they are horribly ineffective. Getting the market to even know your product exists is difficult for a pdf.