Some will do for sure. Gravel for instance already announced that he will quit after the debates, being just in it to shake them up and advance progressive ideas.

Not everybody is in it to win the race.

The first debate is on June 26th and 27th.  They will randomly select up to 10 candidates for each night (no repeat candidates). 

In order to qualify for the first two debates, debate entrants must either:

• Attain a minimum of 1% support in minimum three approved official polls for either the national level or the first four primary states (Iowa, New Hampshire, Nevada, and South Carolina). The polling threshold will be determined using polls published after January 1, 2019 up until two weeks before the scheduled debate, among polls commissioned or conducted by a limited set of organizations (with candidates only able to count one poll by the same pollster within each region towards the requirement): the Associated Press, ABC News, CBS News, CNN, The Des Moines Register, Fox News, the Las Vegas Review-Journal, Monmouth University, NBC News, The New York Times, National Public Radio, Quinnipiac University, Reuters, the University of New Hampshire, USA Today, The Wall Street Journal, The Washington Post, and Winthrop University.

• Meet a fundraising threshold, in which a candidate must receive donations from 65,000 unique donors, with at least 200 unique donors per state in at least 20 states.^[161] Candidates who wish to qualify using the fundraising threshold must present evidence to the DNC of their eligibility using donor data collected by ActBlue or NGP VAN.

Based on the current polling, Sanders, Harris, O'Rourke, Buttigieg, Warren, Booker, Klobuchar, Yang, Castro and Hickenlooper are the only ones that meet the 1% at the national level.  Delaney, Gillibrand and Gabbard might meet the state criteria.  All of the previously listed candidates have met the fundraising threshold.

Inslee meets the fundraising but not the polling while Gravel, Messam, Ryan, Swalwell, Williamson, and Moulton have not yet met either requirement.

If more than 20 candidates eventually meet the above requirements, they will use a polling and fundraising formula to narrow down the top 20.

I see at least 2-3 major declarations (active Senators or Governors) between now and the end of August.

Also, if Biden does declare tomorrow, he's only doing it for his son Beau, who died of brain cancer back in 2015, and told his father he wanted him to run for president.

Ugh, really? Why Gravel? I'd rather have him stay than Gabbard. He's basically everything I like about Gabbard with none of the creepy authoritarian loving baggage, proof that there are better options out there for you if her brand of anti-establishment pro-peace progressivism is what you want. Also he's growing a lot in support and will probably make the debates. He's my number 3 right now.

His son's death was the reason he gave for not running in '16...now it's the reason he's running? Also he's already run for president before, and his son was probably referring specifically to the '16 race. Let's not mince words though. He's probably going to run, and it'll be because the polls show he can most definitely win, and he likes the idea of being President Joe Biden. He has little other reason to run besides that. He has less of a clear vision than even Hillary did. "Avenge my son's death by promoting cancer research" is a cute, heartwarming story, but not a reason to run for president.

Probably due to his declining health. Don't forget he's already 88 years old and needs a cane to walk, and even in his video you can see that he can only barely move. Chances that he wouldn't last through his period are pretty high by the looks of it.

It was too soon in 2016, him still being very scarred by the event and emotionally unfit to run. But that doesn't rule out that he wants to become president to honor him now.

Fair enough. Honestly though, I'm sincerely beginning to think that there are essentially 3 candidates that are truly major candidates with a genuine chance to win. Biden, because he's a poll frontrunner and will have the backing of the establishment, Bernie, because he's right behind and has the biggest portion of the progressive grass roots by far and will probably be the consensus progressive pick in the end, and Kamala Harris, because if Biden flames out with a series of gaffes, Harris is the de facto inheritor of establishment support, because she's acceptable to them but also has some credibility with the progressive grassroots, and thus will be seen as the unity candidate by the establishment, whether she deserves that or not. If anyone other than those three end up winning, it will be because one of the candidates knocked one of the three I listed from that role of the centrist establishment pick, the progressive grassroots pick, or the establishment backup unity pick. Of those, Harris is the most likely to change as she could easily be uprooted by Beto and her star already looks to be waning, though the California primary will give her a last chance. Bernie could possibly lose the progressive pick, but I don't see that happening, his lead is just too strong. Biden definitely won't, because if he goes under, the centrists have no one else with the necessary star power to win, so they'd move to the back up unity candidate and then that candidate would be much more likely to win.