As much as I don’t think Biden should run at his age, you’re basing the “changing the nominee now wouldn’t cost votes” on nothing whatsoever. There is a proven benefit to incumbency. I don’t like it, but it’s true. And the other massive question mark is who does a majority of the country like enough in the Democratic Party (that the Democratic Party actually wants being its nominee) to just install? Because you also have to factor in the mileage the right would get out of “the Democratic Party’s nominee was installed like a dictator!” shit. No to mention the optics of a party elite-chosen candidate. You can’t say a change wouldn’t lose votes without a definitive other candidate to compare to and without considering the optics.
So basically, what you’re saying is mostly just nonsense. This is a terrible situation for us to have to be I . But we’re in it. No use pretending.