Sanders doesn't have a prayer of winning in the general election (his economic views are too far left even for many Democrats, while he doesn't pander to blacks and Hispanic racial grievances enough for them to turn out and vote for him. The Democrats know this and won't let him be the nominee. They'll put everything they have - PAC money, airtime, superdelegates, behind a "safe" candidate as they did for Hillary in 2016.
For a while, we all thought that would be Biden. Since the Democrats are now throwing him under the bus (i.e. these stories aren't just coming up now out of the blue for no reason), this means that they plan on anointing somebody else. For all we know it may be Hillary again.