What if the 5th kyu has been training for 20 years and refuses to participate in the testing process. Would it be inappropriate for him to teach a class full of Sandans who have been training for only 3-4 years?
I'd take his class.
First of all, sandans that have 3-4 years of training, are incompetent in aikido. No kidding. Have you ever seen in your short life a sandan with such experience? You are creating an impossible scenario just to prove your point or what?
If the 5th kyu has been training for 20 years and refuses to participate in the testing process, let him continue his quiet practice in the dark corner of tatami. Teaching class is not his best occupation in the dojo.