There’s no realistic scenario of Biden stepping aside and the nomination not going to Harris. Probably the biggest reason is that Harris is the only other politician who can access the hundreds of millions of dollars in the Biden/Harris campaign account. As I understand ... You can’t just slide that money over to some new candidate under FEC rules.
The Dems should have plans to elevate her anyway in case of Joe having a medical event rendering him incapacitated, or in the event Joe would pass in office. Although I think Biden and the Dems have done her a major disservice by not consistently elevating her status as VP the way Obama had done for Joe. They should have been finding ways to demonstrate Harris’s competent leadership in executing Biden’s agenda, to put her in the most favorable position to pick up if something does happen. Instead she’s been largely invisible from a performance standpoint. Hell, we heard more from Cheney in his “undisclosed location” than Harris.