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.
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.