Maybe it is a matter of different arts are brought together isn't to improve but rather as addition to. Much like how you take swimming plus running plus cycling equals a Triathalon. Hence you have a new sport. I don't think it is a matter of perspective and not improvement. When you add swimming to running to cycling you don't improve upon each sport. You just have a athlete that can do all three.
Any of the people mentioned who did other arts and also did Aikido, they didn't compare to the skill of O'Sensei and they all came after O'Sensei and they learned Aikido from O'Senesi and stayed with Aikido. They didn't stop doing Aikido. I don't know why that is often over looked. They thought Aikido was vaild.