I think they are letting Bidon debate because he won’t be the candidate. They will replace him and state that the new candidate does not have to debate because they already did so.
I think if the dem powers have determined that Biden will drop out, Biden will be the last to know.
If Biden does well or holds his own, he remains the candidate and increases his chances of winning.
If Biden does not do well, the Dems have their excuse to replace him before its too late.
Kind of a win-win for Democrats.