Running someone fresh that appeals to this American Idol-esque popularity contest = good.
What if no such person exists?
Then you just lose and Trump becomes President by default. Do you have confidence that Democrats can rally behind an actually named person? And if so, what is the name of that person?
I’m no Democrat. But I wouldn’t consider “replacing Biden by somebody” to be a serious option. You need to say “Replace Biden by SPECIFIC NAME HERE”. Otherwise you’re just throwing away the election before it even begins.