Again, it's certainly possible that Warren can beat Trump. But that sure looks like a shakier proposition than Biden beating Trump. Every Democrat out there needs to ask themselves: would I rather lose with Warren or win with Biden? Of course neither of those outcomes is a given, but right now it sure looks like Biden is more likely than not to beat Trump, while Warren is IMO a 50-50 bet.
Lose with Warren, get a Dem landslide in 2022 and impeach Trump after taking the Senate.
Taking the Senate isn't enough; you need 67 votes to convict and remove Trump. But even if you manage that, Trump would still have had at least two more years to inflict further damage on the country. And even then you're left with Pence (or whoever replaces him on the ticket) as President.
Would you REALLY prefer four more years of Trump or Trump/Pence to four years of Biden? Wouldn't you rather have Trump out in January 2021?
Some of the Berniebros just prefer to keep a corrupt and inept moron in office with whom they agree on basically nothing, rather than have a mainstream Democrat with whom they agree at least on two thirds of the stuff and who has, unlike Bernie, a real shot in getting this done. I mean, even if Bernie was elected and had narrow senate majority, he still wouldn't pass most of his program in its current form. It's actually kind of sad some one the left are on this #puristorbust lane.