You couldn't pick and choose patients, because no one could sign on those plans anymore. As people switch to other plans, their dwindling enrollment will mean insurer will discontinue them sooner or later.
They've already did pick and chose, when they signed people up the first time. The people who would stay on a grandfathered plan wouldn't be the unhealthy ones who are paying 3 or 4 times what they can get on the exchange. They'd be the healthy people who don't yet know what "pre-existing condition" can mean when you are sick. And they wouldn't move because the insurers would always have an incentive to price those plans below market since those are the only plans they'd be allowed to manage the old way.
When you buy car insurance and don't crash your car, your premiums pay for everyone who does crash their car. That's the service you are paying for. When you buy health insurance and stay healthy, the profits on your insurance pay for anyone who does get sick and keeps their coverage. But a grandfathered plan would be able to keep the sick people out, meaning the profits are straight profit, with much less "medical loss". The open enrollment plans on the other hand would have a slightly sicker average membership due to those healthy people being removed from the pool, driving up premiums for the general population.
The way it works is simple wealth distribution. The members of the grandfathered plan get a small piece of the pie to get them to play ball. The companies keep whatever they can convince those people not to. And everyone else is legally unable to keep from footing the bill in terms of higher premiums. Yes eventually those grandfathered plans disappear due to people moving on or getting dropped. But the companies have an extremely strong incentive to keep them going as long as possible.