A strange thing has happened over the last couple of years, Miller says. Arc has grown fast — users quadrupled this year alone — but it has also become clear that Arc is never going to be a truly mainstream product. It’s too complicated, too different, too hard to get into. “It’s just too much novelty and change,” Miller says, “to get to the number of people we really want to get to.” User interviews and data have convinced the company that this is a power-user tool, and always will be.