Yes, Biden earned the opportunity to serve out his term. He also earned the opportunity to serve a second term.
The thing is, if he is not fit to serve as a candidate, he's not fit to handle the affairs of state. If you're having him step down as a candidate over age-related issues, then he should step down as President for the same reason. Harris then assumes the Presidency, and runs from a position of incumbency--which is a better position to be in than a last-ditch substitute.
What bothers me the most about this is that I can't help but feel that the Democrats caved. The right-wingers said Biden wasn't fit for his office, and this is a tacit admission by the Democrats that they were right. If this had happened during the Republican Convention they would have been popping champagne corks and counting all the ways that they can strip everyone but straight white males of their inalienable rights. I can't imagine anyone among the Democrats breathing a sigh of relief over this.