I'd think it depends on a lot of factors e.g. how easy it is to copy the idea, is the start-up just selling their time, the nature of the customers, the motivat

I'd think it depends on a lot of factors e.g. how easy it is to copy the idea, is the start-up just selling their time, the nature of the customers, the motivations of the founder etc. However sounds like a good rule of thumb, and a good question to ask. "Is this the kind of business model that would get stuck at a salary level income".