So here’s an interesting breakdown from that NYT/Siena poll:
One quick takeaway from this chart is that there are very few people in the “Biden over Trump, Trump over Warren” slice (2% vs. 1% in the opposite “Warren over Trump, Trump over Biden”). That seems more like the “I love herpes” respondents you get when you ask polling questions than like some big trend to favor Biden.
More concerning is the Biden or Other camp (4%), which is twice as large as the Warren or Other camp, and the Other-Other camp, which is a ridiculous 6%. Taken together (and at face value), this implies that Trump gets around 45% of the vote if Biden is the nominee and around 45.5% if Warren is, but that enough people vote third party or stay home that Warren ends up with ~43% while Biden ends up with ~46%. That leaves a lot of votes on the table still who could potentially be persuaded to turn out against Trump. If you figure a lot of the Other-Other voters are third party die-hards, not going to vote, or fall into the category of “I don’t want to support Trump, but when it comes down to it I have to”, there are still 9% of respondents in this poll who are willing to support a Democrat over Trump. Win over those voters (and turn them out) and Trump would have to sweep the table on the undecided/third party only group.
It also included this chart:

Which also has some really interesting results. Biden does significantly better among white college-educated voters, which surprises me even in these battleground states where his “moderate” position is favorable. I have to wonder if that’s a place where Warren and Sanders could make up ground, relative to Trump, once they were nominated.
Warren does worse than Sanders or Biden among white non-college voters, which is possibly name recognition but probably “likability”. She does worse among black voters, but so does Sanders, so that’s probably more based on her leftism. Interestingly, even Biden is doing way worse among black voters than Hillary did, so there’s a huge gap to get up her level of support, and even Hillary’s margin there seems like it was too low (this is all in the battleground states, remember).
Finally, Warren is getting killed in the Hispanic and “Other” slices. I don’t know much about who makes up those voters, but I have to assume most of the Hispanic voters are in FL and AZ (compared to MI, WI, PA, or NC). That could mean that Cuban/Cuban-American voters are lukewarm on her, but I’m unclear why they would like Bernie so much more unless it’s name recognition / sexism. Is there a policy difference I’m missing here? For the “Other” slice, I think that shows she may have some lingering effects of the Pocohontas / DNA debacle, which probably leaves a worse taste in your mouth if your ethnicity doesn’t even get it’s own line-item in polls like this.