The recriminations and panic is just excessive.
What doomed Sanders in 2016 wasn’t only the South, he lost nearly all states with big delegates count, TX, FL, CA, NY, PA, OH, AZ, MA, now more of them vote tomorrow and he’s on track to win the majority of them.
In the very unlikely scenario that Biden trounces him across the Deep South and comes out with similar delegate lead that Clinton did in those places, Sanders would still very likely have a slight lead that’s unlikely to change by the end of the race, given about 60% of delegates will have been allocated by March 20 and the rest are distributed equally if not slightly tilted towards the Northern and Western states in remaining contests.