We've had politicians with clear signs of dementia still being elected years after those signs show up. Ronald Reagan was said to have very clear signs of dementia in the run for his second term.
The only thing that will keep Trump off the ballot at this point is his death. He could be catatonic in a wheel chair and he'll still be on the ballot. There's simply no mechanism the RNC could pull even if it wanted to to keep him off the ballot.