The problem with your argument is that everyone knows Biden is not going to finish his term. A vote for Biden is a vote for Harris. That is baked into the cake so the polls already reflect that. But if Harris were actually the nominee, she would be able to nominate a new vp, and the media would portray her as completely different from Biden. Biden’s record is pathetic, but Harris has no record.
There is a reason why Kamala dropped out before the first primary in 2020, despite having everything going for her in Democrats’ eyes—female, black (or at least having a smidgen of African ancestry), dishonest, hypocritical, sleazy.