If Biden has unique weaknesses not shared by other Democrats they aren’t showing up in polls. Only Harris polls roughly even with him. An open convention is a terrible idea
If you want an Al Gore runs for POTUS in 2024 scenario, you can just have the speculation about an Obama-Gore ticket in 2008 come to fruition. (I don't think it would be a good idea, but McCain-Palin wasn't a good idea either, so bad VP ideas certainly were possible in 2008!)