As to being "without foundation" - well, people from different ages are compared all the time without objective criteria. It's not a yes/no question. What do you think historians do? They evaluate people in the past, many times without solidly objective criteria.
Ted Williams is the greatest hitter of all-time. No drugs, no 350ft. fences, no strike zones the size of a small plate. Yet every year we hear about a "modern" best hitter ever. So yes, we constantly comparatively evaluate others in a manner that transcends time.
No one has surpassed Ueshiba Sensei in the sense that the over-whelming burden of the integrity and ethos of aikido was shouldered by him. Even today, mainstream aikido often relies on the ethos he established. Some of his early shihan would be the most likely candidates in that argument if you wanted to have it.
Technically, O Sensei not only transcended kata, but he was able to transcend the other Japanese systems and solicit appreciation from sister arts. I know of very few aikido people who have done that. Could Doshu look better [than O Sensei] doing irrimi nage? Possibly. Could he look better doing irrimi nage to any student from any other [Japanese] martial art? I am not sure if there is much evidence there... Some of the history folks may have a better idea of that.
Of course, this said, the political landscape of Japanese martial arts has altered, too. I am not sure if it is even possible to transcend martial art systems with the same scrutiny because there are so many more practitioners now.
Oh, and my dad could completely beat up O Sensei.