The strongest candidate for either party to field would be an incumbent President, especially one who has already beaten the other party's frontrunner. They have the advantage of celebrity, a record and the bully pulpit. The second strongest candidate would logically be an incumbent Vice President.
The Democratic Party may have been a shitshow but Harris was the best possible option once Biden was no longer in contention. And the margin between her and Trump turned out to be slim, so a Harris win wouldn't have been impossible.