He would be free to do that. But if the market is sufficiently in his favour that the employer would agree to this (rather than just picking the next CV in the

He would be free to do that. But if the market is sufficiently in his favour that the employer would agree to this (rather than just picking the next CV in the pile), he'd won't need this protection: If fired, he can walk across the street and pick up a new job. Thus, he has a much stronger motivation to negotiate for other benefits such as salary, holidays, remote working etc.