After attempts have been made for accommodations and they have failed or the job description itself cannot be altered. Yes, in that situation it's fine.
We don’t have a diagnosis for Trump, but there is clearly something deeply wrong with him
We know what is wrong with him. He's extremely selfish and greedy. That is what disqualifies him to be a leader. Our speculations about his medical situation are unnecessary.
Disability should not preclude you from work,but just like people without a disability, you seek roles you are suitable for. It’s not ableist to say someone with alzheimers and associated cognitive decline is not suitable for president.
It's not ableist to say that someone with a predictably degenerative disease which is affecting their ability to make logical decisions, that we have no workarounds or treatment for, may be unable to fulfil the job requirements.
It is ableist to suggest that any incidence of memory loss or shitty rhetoric or bad writing or family history is evidence of a degenerative disease and that they should not be eligible for a role because of it. Especially when that conjecture comes from people with no medical background or relation to the person in question.