"Invincible" is not the way to look at it. O Sensei's body methods simply have an integrity that, as a product of that training, impart a high-level ability to absorb, deflect and control the attacks of those who would be aggressors. It doesn't make a person invincible, but it does give one a significantly greater level of protection over larger, violent persons, than do conventional forms of physical conditioning.
I agree that pursuing this discipline is not necessary for everyone. Aikido has developed into different pathways, all of which bear the street sign that says "Aikido." That is the nature of any creative product: in the hands of different persons and different circumstances, it evolves in different directions over time. While some would advocate for staying more closely to the particular physical-development path that Morihei Ueshiba took, there is no cosmic law that states that everyone must. Aikido has taken on a life of its own, and its incarnations provide joy and satisfaction for many.
I do think that the differences of opinion about all this will be the source of division and friction on Internet forums, but in Real Life, it will matter little. Everyone will keep pursuing the path that is right for them, and calling it what they believe it is.
P.S. This column is forever going to be in my memory banks as "The Janet Manifesto"
P.P.S. Happy (belated) Birthday, Janet!