I know many that have left over the last few years, and without exception they are extremely glad they did. I have not heard of a single person wanting to come back. While it is possible the exist, they'd be few and far between. The company have always made it clear that if you leave, it's for good, but I have been told by one or two old hands that exceptions are sometimes made, and the manner of the dismount is crucial. There's nothing unusual in that - many companies won't rehire resignees for the reasons already given. But don't believe for a minute that there are queues of them trying to return.