I personally think the only viable candidate is Kamala, not for her competence, but strictly due to her position as VP. In theory, the party can pick whoever they want at contested convention, but voters would feel severely disenfranchised if they pick someone without involving a legitimate primary and input from single regular person (delegates are chosen party apparatchik, not real people). It would take a lot of corralling, wheeling and dealing to get Kamala, or other aspiring wannabes (e.g., Newsom, Whitmer, etc.) to bend the knee to the party chosen one - they also risk fracturing the base. All of the current presidential wannabes are still losing to Trump, every single one of them (just not as bad as Biden). Furthermore, people like Newsom and Mayo Pete might not want to have just 3-4 months to run the biggest campaign of their life to the highest office, they might wait out until 2028 instead to run a proper campaign. As such, the only obvious sacrificial lamb/viable candidate is Copmala, it is literally her duty to step up if Joe step down.