Biden will give all his delegates to Harris at the Democrat Convention. She will be the D candidate. Nothing else will suffice Replace her and the black women will show you who’s in charge.
If Biden chooses to give up his delegates they are gone—free to do what they wish.
At that point his recommendation (if he makes one) is not totally irrelevant but it can easily be defeated by a determined candidate with strong delegate support.
This could lead to a wild convention—perhaps the old fashioned kind with fifty ballots and smoke filled room horse trading.
Imho Kamala is not a strong enough personality to override ambitious rivals.