Remember Kathy Sierra's concept of a "badass users" ? A user doesn't care about your product, user cares about his context. He doesn't want to be better at using your product. He wants to be better at what he's doing. A user of a digital camera doesn't want to be the best at using this particular camera. He wants to make stunning pictures.
Good news first - a designer is already a PM, a bit. Designers are driven by very much the same forces as PMs - find and solve customers' problems. Unsurprising that many designs are interested in switching to product management to have a greater influence on the overall product lifecycle. This switch, easier than from most other roles , still comes with several challenges, especially when it comes to business aspects of product management.