As a current founder and CTO-by-title of a product company (http://first.io), I think you're missing one path the founding engineer can take: run Product. Whether that eventually becomes a Chief Product Officer or Head of Product or whatever, the founding engineer often has a very strong sense of where the product needs to go and as the organization grows, they ship less code and instead drive prioritization and teams that execute on the higher-level product vision.

At least, that's what I'm trying to do. We'll see how it goes! :-P

Oh, and I have an awesome VPE already, which is a relief!