Obama may have gone to the Senate, but he wasn't in there long enough to rid himself of his rose-tinted glasses. Chuck Hagel and John McCain were flawed, fossilized, sometimes weak, but ultimately honorable men of their time. Mitch McConnell is a cancer who has enough psychopathic insight into the weaknesses of America's democracy to bend it to his will. It's always a risk when electing a younger candidate. JFK got conned into the Bay of Pigs by the military.
If Biden selects Harris as his VP and wins, then later retires and it's Harris vs Lori Lightfoot for the next primary that matters, I suspect from what I've seen of the latter that I'll be a champion for Lightfoot. But in this moment, with Warren having burned bridges with bernie supporters and coming from a state with a Republican governor, I suspect Harris is the best pick.