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For Booker, like many of the younger candidates, this is a trial\test run for a real run later in their political careers. Not to say they aren't serious now (ie like Obama), but I'd bet they would not give up their current job if they had to make a d0-or-die choice. Booker and many of the others don't yet have national name exposure nor the experience of running a national campaign, now is a good time to get their feet wet, make rookie mistakes, so that when they are all in 100% 10+ years from now, they are fully ready and have the confidence they can take hard political hits and not get knocked out.
All-in, now or never, candidates are
Biden, Sanders, Brown, Warren
This is likely to be their best chance, and maybe last chance to give it a shot. Using a boxing analogy, they still have skills to win the heavy weight championship title, but unlikely to be able to do it beyond 2020. We know Biden would have kicked Trump's ass in 2016 (ie swing voters), but the political deals made at the time were made so that HC would get the nomination and he stayed out it of the race.
Most of the rest have have yet to peek, so Booker an those under the age of 60, 1-2 shot left.