There is literally no way for them to have a primary process before the convention.
They have to nominate someone before that convention to make sure they're on all ballots
It's the certain choice because it's the only one.
The only choice MAYBE is Harris, but most of the people calling on Biden to step down are *oddly* (read: not oddly) forgetting this.
Meanwhile SCOTUS said that the president is king, bribery is legal, and they can overrule government agencies.
So hypothetical nominees is not as important