a) any shrewd company would simply pay key people more – and, indeed, good pay and retention bonuses are common for people who are truly valued by most companies (senior management, top sales people, etc.) but that very rarely includes developers. The fact that many developers self-identify politically with the CEO doesn't mean their boss shares that belief.

b) that company's value is based on a mix of value which they created and where they benefited from prior art. In the former case, copyright law would prevent John from taking source code and patents would prevent him from taking novel work. In the latter, passing on a trained employee is no more unreasonable than it was for them to benefit from the long history of work which they also benefited from without having to pay for it.