If you are designing for someone, you have to base your design decisions on what you have observed that person doing or saying. By your logic, generalizing from

If you are designing for someone, you have to base your design decisions on what you have observed that person doing or saying. By your logic, generalizing from past observations of other people is never going to be enough to make an adequate design. So either you have to literally prototype each touchpoint (for example) with every possible customer (even the ones who can't articulate what they actually need), or the design is going to have to be based on insight into human nature and (varying) capabilities. Luckily, most designed things are simple enough that generalizing about users in order to discover the "right answer" isn't an unachievable fantasy.