Normal
On the reversibility point, I think it is much more to do with that few people look back even when they have enjoyed working for a previous employer. Whatever the role, or the move between organisations, people don't tend to go back to previous employers in huge numbers. I don't think it has anything to do with the employers' attitudes, I think it is much more to do with the mindset of the individual employee. They left for a reason, and in more cases than not that reason will still be there. Sometimes the grass isn't greener, but it usually just makes people look forward to the next opportunity rather than look back at the one they gave up.