Can you elaborate?
There are loads of reasons why someone would rather work for Big Corp. than doing a startup, especially when they've already checked "build a successful company" off their bucket list. I can only speculate, and I don't feel it's right to in what was probably a very personal decision.
I mean, is that a good way to "leave a legacy"? Why not keep trying on Milk?
If Rose goes into google+ maybe Google wants him as a focus point or part of a public image... these are what I see Kevin Rose as with this but I of course may be wrong.
I don't know however how you got "anyone that hasn't heard of Google has heard of Kevin Rose" from my statement however.