Of course anyone is replaceable but some knowledge will be lost. Also your model does not account for businesses which are aggressively expanding and doing a better job than the average employer retaining their employees by either being a genuinely great place to work or LTIPs
* People who have moved, and now want a job closer to home if they want/need to go into the office.
* Companies who have hoarded cash during the recession and can now offer big salaries.
* Looser bonds with colleagues they only see remotely
I've no idea whether and to what extent any of these things are true, but it's definitely easy to imagine a big move around. That said... I'm not seeing much change in volume on the typical job boards so perhaps it's all hypothetical.