You're simply wrong... not surprising.... perhaps you should pay more attention to what Democrats... and the rest of America and the world... are advising him to do... he's a danger to everyone but most concerningly himself... you have to know when to fold them...
If you are so certain Biden will drop out, you should promise to vote for him if you are wrong.
The reality is it would be nearly impossible to replace him by now. The campaign money has already flowed into his campaign. To run a challenge to Biden, a candidate would have needed to be in the race already for nearly a year. There is no Democrat setting himself up to be a replacement for Biden, because there is no outcry for a replacement to Biden.
Meanwhile, there are a dozen Republicans looking to replace trump. They have little chance, but they have enough of a chance they are willing to risk their careers on it.