That's exactly the problem. Castro and especially Warren would have outshined Clinton. You don't want your VP to run with the attention.
I think it's a good choice. A safe pick that Clinton knows well and apparently they have a strong professional relationship.
But that said; things are different this tiem around and if Clinton thinks she just have to do the usual and run a campaign on being the 'safe pick', I think the election might become closer than comfortable.
'cause holy moly Trump is problematic. Latent dictator in his rhetoric and thinking. Me, me, me. I, I, I.
Dude, talks about fighting crime as if it's something the President is responsible for. How many police departments are there in 'murica? Around 18.000