The bigger the field, the smaller the number of committed voters that you need to win. I read about a Sanders advisor that noted in 2016 Bernie needed to get 51% because he was (virtually) the only other candidate than Hillary, but in 2020 he could easily win with 33%. That's a little exaggerated; he would clearly need to pick up a share of the voters for candidates who drop out, but certainly in the early states, if he can get 33% he should be winning. Granted, because the Democrats don't do winner-take-all anymore, it won't be worth as much in terms of delegates, but that will be more than made up for by the media attention that goes with being a winner.
On the GOP side, I could understand the idea of running one time to get the voters acquainted with you, and the second time to get the nomination. If you look at the history of the GOP nominees who were not sitting presidents, they tend to be second-timers for the last 50 years:
1968: Nixon (previously ran in 1960)
1980: Reagan (1976)
1988: Bush (1980)
1996: Dole (1980 and 1988)
2008: McCain (2000)
2012: Romney (2008)
Obviously GW Bush and Trump are exceptions.
The Democrats tend not to go that way, and when they do they lose. The Democrats who have won the presidency in my lifetime (who were not sitting presidents) were all first-time candidates: JFK, Carter, Bill Clinton, and Obama. The retreads are Humphrey, McGovern (he sort of ran in 1968), Gore and Hillary.