[clavalle]

>If the replacement takes 6 months to get up to speed the replacement is certainly not a very good developer

I think you're mistaking 'becoming a contributing member of the team' for 'contributes at the level of the developer that left'.

[SideQuark]

Often the person leaving has not been that good of a contributor due to wanting to leave, while a replacement is new and likely more inclined to work hard.

On a big team, people fit a bell curve, and most likely those leaving are not going to cause much harm (otherwise no big project would get done - all have people coming and going over the lifespan of the codebase).