It's tough to do this in the modern day because for them to have enough worthwhile experience they almost have to be in their late 40s. There are a few generally accepted routes to the presidency and legitimacy in them puts you at a fairly old person. You can go the executive route, someone who was a governor or military leader, or you can go the legislative route, Senator or Representative, or the third path of being generally unqualified.
Obama is a good example, he had a career (law degree, became an attorney, taught at the University of Chicago Law School, got married, had kids) before he then ran for the state legislature at 35. After being in the Illinois Senate for 8 years he ran for the US Senate and held that position for 4 years before being elected president at 47.
You're too hung up on the terminology. My point is that they need to put someone up now and begin pushing them, by the time we get to the primary that person should be such a forgone conclusion that the primary is barely necessary. This probably means the DNC would pick someone and it'll probably be a shit choice for leftists, but the alternative is that we get to the election and they are tripping over their own feet like the last couple primaries.
I'm not fond of Newsome either, but I would much rather have him than Vance or somehow Trump. The last thing I want is for the Democrats to fumble the next election because they weren't organized and tried to decide who to support 7 months before the general election.