Sure. Just like anyone can be a doctor, engineer or chef if they have a list of clients (or "users") who've been satisfied by what the designer offers them. This applies to any job.
But few question the authority of a doctor, engineer or chef, in comparison with designers. Why do most people feel like they know better than the designer?
No, a lot of people question doctors, engineers and chefs. Some have even made a literal living out of it... Example: "Second opinions" and "food critics".
Sure. Just like anyone can be a doctor, engineer or chef if they have a list of clients (or "users") who've been satisfied by what the designer offers them. This applies to any job.