You can't stop people from leaving. Look at USSR, India or any other ex-socialist economy. Unless you turn your country into a kind of North Korea, these things can't be stopped. And even if you did turn your country into North Korea, the rich will likely be in the top party positions and do whatever is best for them.
Every other socialist economy has had this thing in the past to make it hard for people to leave. They don't work because you can't make people stay against their will.
Most countries have now learned this the hard way. On the longer time period you can only encourage people to do good work, you can't punish them into doing it.
Ah ok I think I get what you're saying better now. Once people decide they're leaving and never coming back, whatever rules you have in place are worthless since they're going to ignore your exit taxes or whatever since they're not coming back and they think your regime is illegitimate anyways.
Every other socialist economy has had this thing in the past to make it hard for people to leave. They don't work because you can't make people stay against their will.
Most countries have now learned this the hard way. On the longer time period you can only encourage people to do good work, you can't punish them into doing it.