I don't think he's unable to effectively communicate or rapidly deteriorating. Nothing I've seen since the debate counters that opinion. The only thing he might not be able to do is beat Trump. There are no potential candidates for whom I can say otherwise, so I will stick with the incumbent.
Rapid decline? No. It's been 4 years. He's visibly older. He doesn't seem mentally incapable. He also claims he's sick. If he recovers & improves, that will strengthen my position. If he does not recover, that will strengthen your position. We shall see. In the meantime, he should keep campaigning.
So far I haven't seen him struggle nearly to the degree that he struggled during the debate, in a variety of settings & formats. If he can recover, I prefer to retain his incumbency advantage.
I'm not confident that he's losing or winning. I do not find the polls trustworthy enough to give much consideration to them beyond their movement. I do not think a candidate trailing in polls is cause to abandon a presumptive nominee with an overwhelming delegate majority, against all convention.
I don't believe you have any evidence to substantiate that risk beyond what I'm already aware of. I think you're seeking a safety that isn't on offer. I don't believe there's any candidate who's substantially more able to defeat Trump, & I am certain no such candidate has presented themselves.
I know what you believe you are seeking. I disagree with you, & I am judging you mildly. Likewise, it appears to me that you judge me & people who agree with me as dangerously complacent, or even willfully blind. I'm not seeking persuasion here. I just want everything to be as clear as possible.
The words you're looking for are "campaigning" and "hard work".
Nothing is ever guaranteed. But Biden being as senile as he is places a sharp limit on what he can do.