I lieu of his sudden passing, Biden was allowed by the Democratic party to be too proud to step down because of his legacy. They can't force him not to run, but they can allow him to have a go and show the public himself that he is not fit for the job. 

IMO Someone with high energy and power who's a real contenter is likely to take his place at the last minute (ie soon). This way he doesn't get defeated, he just couldn't compete etc, keeps his legacy as the standing president, allows him to save face, and will deploy the art of surprise as it will throw off the competition (they expected Biden).