Exactly.

I don't think Biden has lost control of his mental faculties. I think he is struggling with the power of speech. I think he has a hard time quickly and concisely articulating his thoughts. I would bet that he is still a decent (but slow) writer.

But you can't do the job of president by email. You must be able to communicate, quickly, accurately, and under pressure.

I don't know when my grandparents (all four of them) actually transitioned from difficulty speaking to significant cognitive decline, and that's the problem with Biden. At some point, he will simply be following his caregiver's prompting rather than actually making a conscious decision, and we won't know exactly when that has happened.

Trump, on the other hand, is more like my SO's grandmother. She could form full, complete, mostly angry sentences, but they were lacking coherent direction or intention. She could communicate smugness and anger all the way to the end, but rationality began to disappear years before she passed. Everyone knew she was senile except herself.

Trump is well into his own cognitive decline. He's been able to mask it pretty well because he's always been an insufferable, narcissistic prick, but his smug anger has largely become autonomic reflex rather than conscious intent.