This is why it's helpful to work in a big, successful company before founding your startup. You get a sense of the scale and complexity of problems that actually have real users, and practice dealing with those problems. If you stay long enough, you also get a sense of the trade-offs involved, and which complexities are accidental vs. which are inherent.
Of course, when you actually go to found the company, you should forget all that scalability nonsense and start with the simplest system possible. However, it's worth having an image in your head about how the system will evolve and what the appropriate response to various scalability challenges is.
