Let's say this is true. Let's say that Republicans are concerned about Trump's handling of the Coronavirus crisis. Let's say that he didn't handle it perfectly. (That is a very HIGH standard by the way. One that they never would have held Obama to). Okay you voted for Trump, but now you are skittish.
Do they think that even given that, ANY onetime supporter of Trump would vote for Biden? I don't even think that staying at home would be likely.