No Democrats are deciding Biden’s not up to it. Only Biden would decide that. Or Jill if something happened to him. Even then Michelle Obama has no campaign infrastructure, no campaign money, but more importantly, has never expressed an interest let alone a desire to be a politician.
I’d also love to see the source of what you’re reading. Because if this is what the GOP hopes, and that’s where this is coming from, my conspiracy theory would be that their internal polls have found no way to defeat Biden.