If Hillary match Biden's splits with independents, she would be president. Turning out your base and convincing swing voters are the two most important. But I don't think her lost can mainly be put on her lack of appeal with young voters.
Young voters are too fickle and vote in too low a rate, to be the key to electoral victory.
The "young people would save us" way of thinking have sunk many a Dem.
Dude you are using one of the best political talents in modern history to argue that Newsom would be appealing.
Obama did turn out young voters above what you would expect a generic Dem but Obama also over performed in many other areas in 2008 (even though he ran behind the party on the generic ballot).
I just don't see Obama level talent or appeal in Newsom
Plus if Newsom ran in 2024, he would be a decade older than Obama when he was elected.
A 56 year old career politician that was major of San Fran, governor of Cali, with political scandals in his past, whose Republican ex wife could manufacture a negative headline about at any moment just doesn't seem anywhere close to having Obama level appeal.