IMHO Yang and Gabbard were the only credible Democratic candidates.

Yang who? Seriously, I'm not sure many people knew anything about him or that he ever had a chance. The Democrats have their shortlist of acceptable candidates and generally make promises to the next candidate years before. Look at Clinton, she was promised she would be the candidate after Barack if she backed off/dropped out and supported him to win. Look how Bernie was burned in order to fulfill that promise. (maybe that only works if your nickname is Killery and everybody is afraid of commiting suicide in their sleep.)