1. How is that a concern about HER?
2. All of her voters are not highly educated.
3. Not all of her voters are going to vote dem regardless
1. Fam, anyone who subscribes to the idea that this election will be a turnout based election ought to be open to the idea that the best option will be whoever turns out the widest variety of voters. The person with the broadest reach is the best option under that supposition. Now you could argue that for everyone Sanders brings in, he'll lose the same amount. Or that Warren's ability to speak to and distill policy in a relatable way more than makes up for that concern. But if we look at polling data, there's no lie here.
For both 2 and 3, I'd argue that you're speaking to a minority of her support. While the statement being pushed is one that applies to the broad majority of her support.
2. We've got plenty of polling on what occupations support each candidate. Obviously it's not 100% highly educated. But the biggest donor demographic and the biggest section of her support overall comes from the field of educated. There's nothing wrong with that. The only concern here is that, highly educated people have been a part of the party for a while. So if we're concerned with getting more people to show up to the polls, then we'd target new voters. It's a strategy argument but not an insult to anyone.
3. Of course this is true. Not all voters for any primary candidate ever will vote for the person that beats their preferred candidate. But according to historical precedent it's rarely above 10% of those primary supporters (exception being Clinton supporters when she lost to Obama). Also, I think that the loss to Trump and his past 4 years have made it even more likely that a vast majority of people who vote in the primary will make sure to vote for whoever wins that primary. I fully admit this last bit is speculation.
Here's my thing though...none of this is an insult to Warren supporters. There's a purposeful comment made at the front that she's our second choice. That also does not apply to all Bernie supporters, but the vast majority of us recognize her value and support her as the next best option. The idea that Bernie brings more newcomers to the party and can have a bigger impact is the safest difference you can draw without attacking her or her support. It draws a simple distinction in Bernie's ability to appeal to Independents (He's not a democrat right?). And it manages to both acknowledge Warren as the second best option should Bernie drop out while also making an appeal to consider him the more electable candidate. What more could you ask for from a competitor in this race? What is he supposed to say? He's not gonna have "vote for Warren, that's fine."