She’s correct, there are so many complex factors involved in changing candidates in the middle of an election cycle. For example, the only person who can legally use the money Joe Biden has raised is Kamala Harris. Anyone else would have to start from scratch, building resources of money and people.

That's a misunderstanding that's been going around on social media and repeated by some journalists and politicians. But the Biden campaign can legally give those money to the DNC, which could then dispense them to a new candidate. This article debunks this and other such claims:

There’s Only One Reason Biden Won’t Drop Out

His campaign’s rationalizations for the status quo don’t add up.