Apparently, the Campaign is "testing" how Kamala would do against Trump. I'm not sure that would be good for her, though. Or, rather, her political future. Is it worse to be a running mate on a losing team, or the principal on a losing team?

Even if they don't make any changes to the ticket, that's still very useful information to have. Even before the debate, some voters would have at least weighed the likelihood that Biden wouldn't last in office another 4.5 years. After the debate, that's probably a pretty large segment of the electorate. So the Biden campaign probably needs to know how comfortable voters are with the prospect of a President Harris compared to a second President Trump, so they can take that into account in their messaging.

Harris is better off being the running mate on the losing team, rather than top of the ticket. It would be an unusual situation, and unfair to her, but I don't think someone could recover from losing to Trump and convince primary voters the next cycle that she's capable of beating whoever the 2028 GOP candidate will be.