Harris already earned it by being elected, so it's not about letting her earn it. The choice is Biden's alone to make, and it's obvious that it would politically amplify his endorsement and significantly reduce the likelihood of further drama within the Democratic Party so close to the election. Moreover, it would likely mitigate any legal challenges related to Biden's transition to Harris.
This argument seems to be missing the point. The discussion is about what Biden should do, not about Harris's qualifications or the primary process. Stepping down would give her a significant advantage and make her presidency more likely, which is presumably what Biden wants. It's a logical step to support her candidacy.
