Biden has to stay in and he will probably win the election. That's not the issue....it's what happens after he becomes President and slips down hill .....hard. His health(image) doesn't fit his age that well...but he can not help that.
And why on earth is that an issue, should it happen? That's why we have a vice president.
I'll point out again that in the last century we had a rather large number of presidents who became seriously ill during their term in office: Wilson, FDR, Eisenhower, LBJ, Nixon, Reagan. And yet we survived. The last century was our great century.
Are you watching his speech in Michigan right now? He's on fire. Really.