I know that Sanders pulled off an upset in 2016 while polling badly, but I can't imagine the polls being this far off now. 2016 was also a different race and Hillary was much more unpopular than Biden.
Many Sanders 2016 votes were merely protest voters against HRC, who did poorly with WWCs in rural areas. Biden has done well with them in ST primaries. So this isn't a factor anymore. The same happened / will happen in Appalachia primaries such as WV and KY, though I doubt Sanders will be in the race by May, when these states vote. If recent polls are remotely accurate, Sanders is done by the end of March at latest.
