Before midnight musings: every person that leaves, leaves with their vision.
This is the same as products copying other products. You can copy features, but not the vision of the people who made them. So even in an array of products that do similar things, those that show promise have a strong vision behind them.
So for every person that joins a team brings their vision with them. For every person that goes, takes their vision with them. For an individual, where would that person bring their vision to?