I'm shamelessly stealing an idea for this topic from Sphinn because it's a quite interesting question.
I do wonder if it's really possible to quit SEO entirely if you're in web business, but the point of SEO is to get traffic so there are some other possibilities. One of the answers in that thread mentioned PPC for example, which I suppose can be made sustainable if you make every paid click count. Other possibilities may be various kinds of direct marketing and social marketing. Instead of a typical content publishing model you could be in the business of creating and maintaining a web application which would mostly rely on some paid advertising to get things rolling, and then viral and social marketing to keep them going (if the app is that good).
So.. has anyone thought about this? I'm not advocating quitting SEO (despite not liking a lot of it much), but it seems like an interesting question.