My hope is that the field gets whittled quickly, and that the candidates spend as little effort as possible trying to destroy each other, rather than presenting positive ideas. Bernie-ites are already trying that with O'Rourke, and deceptively, too. That does not bode well for the actual party (which Senator Sanders is not a member of). It's ironic, yet unsurprising, that the most vociferous (and disingenuous) Bernie defenders aren't even Democrats, either. Why the Bernie Movement Must Crush Beto O’Rourke (Jonathan Chait, New York Magazine)
And, Chait notes, Beto's critics don't represent most Democrats; they don't even represent typical Bernie voters. They are the most left of the left-wing supporters. The threat that they see O'Rourke poses is that a) he is too "mainstream", and b) he does a better job speaking to minorities and women, and might engage the Obama coalition.
Personally, I think Booker and Harris are the greater threat to Sanders voters. They just can't see that yet. What most Beenie voters don't notice is that Bernie won whites, but not women and minorities. O'Rourke can do both, which is why he is a threat.