for me the Biden conversation is entirely about whether he can win in November. i’m not worried about whether he can actually do the job because i don’t think it’s a real job.
I think if he's in cognitive decline he may have more trouble convincing people he's not which will make it more difficult to win in November, but I agree- Reagan already proved a President can run the country effectively with Alzheimer's, so I don't see why I need to worry about Biden there.
Reagan really losing it actually improved his Presidency since it let Nancy pull off a palace coup to depose arch-conservative and Iran Contra participant Don Regan from the Chief of Staff role and replaced him with (relative) moderate Howard Baker