Now when talking about a stellar figure like O'Sensei you have to differentiate even further. This man was on a path, a lifelong path, searching for something he believed he could find in martial arts and when he found it thus was borne Aikido.
A different level of character and purpose so using him as an example is not comparing like with like I'm afraid.
Note that thereafter it wasn't a matter of him going to other arts but more a matter of him developing his own and hundreds if not thousands coming to him, drawn like magnets to these new principles that he could demonstrate yet they couldn't understand. A new learning.
This rules out the view of mixing practices in order to improve as a bland statement.
Pure rubbish and an incomplete thought process from the start.
None of what you say is factually correct or even logical to anyone who has the slightest knowledge of Ueshiba's training experiences and research. Your founder did everything you are railing against; it was exactly his going out to other arts in order to
develop his own. He continued to research and continued to develop.
You see his art as a finished product, and that he knew best for you... he obviously never thought that way or even took on that role. He encouraged people to develop their individual way of aiki.
Were I you, I would consider that genius is not stagnant. Followers of visionaries rarely do as well. They lag behind and never catch the original fire. He knew it and tried to break his own mold for others to catch on.
The majority of followers are not visionaries, instead they opt for a pallid substitute and sadly think it is all they are personally capable of and keep pointing to the past to validate their present state. It's easy not to excell when you convince yourself that you can't exceed the model you placed before you.
Aikido can be so much more than it has turned into. It can be one of finest arts the world has known, but it needs a serious shift in focus to do so.