So you think Biden at 77, and given his experience as VP, is going to want a purely ceremonial VP?
The most likely scenario is that Biden runs things similar to how Obama did.
Even if we assume that Susan Rice is the strongest candidate on foreign policy, she is still going to be a lightening rod for attacks and she's unproven as a politician. Even if we grant that Biden will still win no matter the VP (which I don't think is a given), it is about the sustainability and his ability to get things done.
Biden is also picking an heir apparent, and it needs to be someone that American's can imagine taking over from him. He has to be thinking about political viability for the mid to long long term. I just don't see how political considerations aren't center stage no matter the polling.