Morihei Ueshiba, prior to developing his own art, had mastered other forms of combat such as daito ryu and even sumo when he was younger. It was from this solid base--not out of thin air--that he then developed his own interpretation of martial arts in the style of aikido.
"Even though our path is completely different from the warrior arts of the past, it is not necessary to abandon totally the old ways. Absorb venerable traditions into this new Art by clothing them with fresh garments, and build on the classic styles to create better forms."
Myself, I study japanese jiu-jitsu, but glean from the traditions of karate, boxing, and, of course, aikido. I'm dressing up my own training in fresh garments in order to create the best forms for myself.
And if Ueshiba were practicing today, no doubt his style would evolve to meet today's nuances. But of course, it would again be a tweaking of the "old ways".
And ultimately, the Master would repeat the one thing that has transcended every epoch of Japanese warrior history.
That is, "Illuminate your path according to your inner light."
This is the true legacy of Morihei Ueshiba.