I agree that Harris was the only choice given how it went down, but if the DNC and Biden could have come to terms a year or so prior, it would have provided enough time.
I go back to what I said as soon as the switch happened, she was a terrible candidate. She, perhaps somewhat unjustly, was going to get pinned for everything wrong that happened under the Biden Administration (though her role with the immigration issues was meaningful). But, she was god awful in the DNC primaries in 2020. She got little traction and wasn't even a Top 5 primary candidate, dropping out in late 2019, well ahead of others.
I'm also not entirely convinced that any Democrat wouldn't have been able to win, assuming it went down differently (standard primary) and a more moderate candidate was elected. Kamala was the second most liberal senator according to voting records for the 115th and 116th Congress.
I don't think Shapiro would have won, for various reasons, but someone of that ilk who is seen as more moderate certainly could have helped.