I think Biden had a low chance of winning vs Trump based on recent events (bad debate and what happened to Trump at a rally). Biden was all in before those 2 events.
The switch up was necessary and the easiest path was Harris as she can spend the money. The important thing will be her VP pick. I think switching out Biden gave dems a better chance to beat Trump.
Let's be honest, the dems will vote dem and republicans will vote republican. Even if republicans decide to switch to a new person right now they'll vote republican, same with dems.
It's the battleground states that matter the most, MI, PA, AZ, etc. If anything Harris comes off as a moderate because she's was a prosecutor.