In my opinion, there are some people that are supposed to be aikidoka, and there are some that are meant to do something else (at least for a little while). If someone is seven feet tall, they'll probably be drawn to sports like volleyball or basketball. They might have played a few games of baseball and decided that it was "stupid", or just not for them. To each his own.
There are many ways for people to defend themselves. Skunks, porcupines, and grizzly bears are all not to be messed with, for a variety of reasons.
As we go through our lives, our ideas of what power is might change.
O'sensei was already a tough guy; why did he choose to change?
I think that when I'm a little older, and not able to rely on strength so much, I'll be glad that I've practiced aikido. I study tae kwon do in addition to aikido because it's fun, and I get to hang out with my "martial arts family" for a couple more hours each week. I'm also learning skills that I believe have value.
I think that as long as we strive to make forward progress and try to improve ourselves, it doesn't matter very much which art we study. I think that we'll probably be better off just for making the attempt.