I don't get how that is Osensei's opinion on a grading system that was largely being built by his son at the time. He mentions mastering the basics and taking at least ten years to reach the "first rung" - not first dan (I wouldn't mind seeing the original Japanese that John Stevens translated). In any case, the founder handed out plenty of third dans within ten-year periods and shodans even sooner. Saito sensei also said that one doesn't master the basics until sandan. It seems to me the founder's warning was to the student focusing merely on rank rather than attaining the basic skills of the art.
I think it would be easier to interpret if I could find the quote in the original language. I think it was more of a warning for people who only care about rank.
I know in my federation it takes between 6-10 years to reach shodan.
A teacher told me that shodan meant that you learned enough of the basics to finally start learning aikido.