I would expect the majority to continue with the current system, but it surprises me that (if it's not about money but rather is about complexity) there aren't doctors opting out.
The same reason all licensed <trade> in your area are about the same price for the same work.
They paid years of their life into a system that lets them bill exorbitantly. They're not gonna undercut it. And if they are they're only gonna do it enough to get enough volume to keep them busy, which isn't much.
And while most people think of going to a doctor's office - family medicine, internal medicine, pediatrics, or OB/GYN - as what doctors do, they're actually a minority of doctors, and OB/GYN's do a lot of their work in the hospital. Some of us - I'm an anesthesiologist, but also radiologists, pathologists, critical care doctors, and so forth - don't have a clinic at all. Nobody's going to pay me a monthly or yearly fee, and establishing a billing relationship that doesn't involve insurance would be a real nightmare.