You answered those two questions as if the doctors being discussed in each question are teh same. Of course, they are not.
Could some doctors who currently see Medicare patients opt out of all insurance and only care for the rich? Sure, a very small percentage of them could. If this percentage is over 2 percent of the doctors who currently see medicare patients, it would be a huge shocker.
The remaining 98% could not do this simply because there is not enough demand. In fact, it would be VERY surprising if there was enough demand to allow even a fraction of this very small percentage of doctors to do this.
This leads us to your second answer. You are hypothesizing that all good doctors will stop accepting medicare. I have seen NO data that suggests that this will happen. In fact, it is about as likely to happen as all doctors leaving any major commercial payor's plan. It's not that it isn't likely to happen, but it is that it is almost impossible for it to happen because the payor would simply adjust its payment methodology and reporting requirements to stabilize its network.