Another option: they replace Harris on the ticket with someone that is popular, unlike either Biden or Harris. The new running mate becomes the clear president-in-waiting if elected which makes them the actual candidate. Even less friction since replacing the running mate is easier.
I misread that at first, but here you're still running into the "Why should Harris step aside?" question. If you replace her with another Black woman you're clearly just pandering to demographics while insulting those same demos, and every other choice raises worse questions.
That's the kind of thing that happened routinely when I was living in Brazil, except folks also had a dozen parties to jump into and out of. It's a bad look, to say the least, and it never accomplished much in terms of actual governance.