Exactly! Sounds like people really need to watch Heather Cox Richardson's recent CNN interview. She talks about this in some detail, and historically, a late replacement after the electorate has chosen its candidate will lose. It's a sure-fire way to hand the election to the other side.
Just look at the 1968 election - Johnson declined the nomination after being selected through the primaries, and switching to Hubert Humphrey gave Nixon the presidency. We all know what happened after that.
https://www.cnn.com/2024/07/03/tv/video/amanpour-heather-cox-richardson