A lot of students have no desire whatsoever to use Aikido as self defense, and personally, I would rather have soeone who understands its principles and can demo them on me teach me than someone who looks like a Marine, but has at best a rudimentry knowledge of the principles, rather just knows (vaguely)the steps in a technique.
I saw a woman in a wheelchair do a great demo at the Aiki Expo, and was lucky enough to train with her during one of the classes. Someone else mentioned a sensei they had who had a stroke. I can think of at least one 7th Dan who rubs his hip and back repeatedly during classes; most of us wouldn't turn down a chance to take classes from him.
Those who need their instuctor to look fit, what ever that is in their mind, fine for them. As I said, some need (especially those doing it for self defense) to see in their instructor what they wish they saw in themselves. They will find some big, tough, fit guy. I've had several instructors like that, and they were great.
Others may be just as happy (or happier) with someone who looks like they do, whether it is older, or smaller, or with that 'sensei belly' that holds a hakama in place so nicely. I don't care if they can't make it through a class as a student; some of the older ones who've smoked a lot may not be able to, but who cares? (I care about their health, of course, but if they've smoked themselves into COPD, the damage is done) Can they teach me what they know, that is the important question.