There is no Plan B in the centre-left/moderate lane, and enough Sanders supporters (NOT speaking of the DU ones) will never switch and vote for Warren in numbers large enough to make up the difference. Harris had a chance to perhaps make a play for the middle, but she instead went hard left (why, I do not know) and is now in a diminishing limbo of not being favoured by the left and has alienated the potential pulls from Biden's lanes with her ill-informed attacks on him.
The nomination is Biden's to lose as long as Bernie stays in. The only way I see Warren having a shot at it is IF Bernie dropped out and wholeheartedly endorsed Warren. That will not happen. Many of his supporters will not listen to him even if he did that. He is Biden's best asset for the nomination (along with the penumbra of the Obama years), which is why I find it so curious that many of his supporters just tear into Sanders non-stop. If Warren dropped out, there would still not be enough of her support who would slide over to Bernie for him to win either.
The only way I see Biden not winning the nomination who be a complete systemic collapse or some major health issue appearing out of nowhere. Both of those are extremely unlikely.
IF Bernie had truly cared about having a progressive POTUS, he should have never run this time and jumped on the Warren train from the start. I do not think him capable of such an act. He projects an aura of 'It is only ME who can save us'. That is not a message that the country needs right now. We already see the results of that type of behaviour as it is.