I say this not as a way of poo-pooing his chances out the gate, but the most common argument for Sanders (how well he did in 2016) in contrast to his polling numbers exposes something about his candidacy. Yes, he sponges up a good chunk of genuine leftists, but also much of his support just came from being the only viable not-Hillary. Webb and Chafee didn't belong anywhere in a Dem primary and O'Malley did nothing to distinguish himself from a generic D politician.
I think this is very important and needs to be stressed again. In 2016, Bernie's coalition was made up of a few different prongs. There were the true believers/far left, the anyone-but-Clinton people, and then young voters who were into the next big thing. (Obama benefited from this group in 2008 bigly.) There was also a not small subset of people who supported Bernie because he was a white male. (No, I am not saying his supporters are all racist, white men. But, you have states like WV that swung like 30 points away from Hillary. These are places she won in 2008 because the other option was a black man.)
These groups, imo, no longer exist in the same way. He still has his true believers, which I very much believe are around 10-15% of the vote. But he will not benefit form the "anyone but Clinton" voters because they will have options. There are fewer caucuses which were the majority of his wins. Plus, he is no longer the new hotness. In addition, he has shown zero ability to reach out to black voters, and has made zero attempt to increase his stable of surrogates. On top of this,. there were a lot of folks in 2016 who thought there was no chance in hell of Trump winning no matter what, so they were willing to roll the dice on someone like Bernie whose General elect-ability wasn't forefront. I think 8 out of 10 dems main thought is going to be who has the best chance at winning. (This is something we saw with Obama/Hillary in 2008. Once people saw that Obama could actually win, Hillary's support started to erode a bit. Also, 2008 had a lot of Bush-Clinton fatigue. People complaining about the whole "dynasty" thing, even though Hillary was experienced in her own right, but whatever)
Bernie, like Biden, is at near 100% name recognition. The idea that his support is so soft it can just deflect to other candidates, and he can get it back if/when he announces is not a great argument imo. He has Warren who will attack him from his left flank, he has Beto who is the newness, and he has still done nothing to ingratiate himself with the establishment who, like it or not, is the vast majority of the party.
Also, I do not think he is a good fit for the 2020 electorate. There isn't going to be a single serious candidate who isn't going to be campaigning on things like M4A, "free" college, $15 minimum wage, etc. You don't get credit for having the idea first, it's just who has it best and in the best package. Also, he has a ton of baggage that no one has ever touched on a national stage. His stance (and comments) on guns will be used heavily, as will his vote (and cosponsoring a bill) to send nuclear waste out of Vermont and to a poor Latinx community in Texas. That's not even touching his praise of Castro (which will doom him in FL again), the whole "went to the Soviet Union on his honeymoon," his wifes problematic involvement in falsifying documents to get a loan for her school....these things will actually be brought up this time.
Plus, the calendar in 2020 isn't friendly to him.
And to be clear, while I loathe the man, I would vote for him in 2020, but I would literally complain the entire fucking time.