I've done aikido for 3 years and have never come across a sensei telling me that "moving the pelvis" is fundamental in aikido, nor has Ueshiba, Shioda, Tohei, etc. have talked about it. You are talking hot air here.
No one is saying that. You brought that up. Here is what I said, "Sumo can teach us, that proper body alignment (support your local chiropractor ) is essential to Aikidoka's. Don't believe me? Watch films of O'Sensei when he was young and when he was old. Look at his hips and small of his back, note what they do and what they don't do. Then do the opposite in the dojo at your next practice. You should find the importance of proper position has to do with getting that connection and controlling that center in your Aikido technique."
I am talking about body alignment. I am asking to look at his hips and make an observation. Then I suggest to take your observations and test it the dojo and see what the results are. I suggest that what a person find is the importance of proper body position in controling the center mass of the body. That is a very fundemental concept in Aikido and many Japanese martial arts to move from your hara, where you power comes from. If you want that to work at a optimum level proper body position/alignment helps. Find that is up to you, and it is done via practice.
You asserted in the OP as though you clearly knew what you were talking about, so I wanted to inquire on the depth of what you definitively asserted.
As I said before, I don't claim expertise or have the answers. That is up to each individual to find their own answers, and conclusions. I have shared mine.
My POV is a method of personal discovery and sharing information and that is what interests me. That is the beauty of a form, and in IMO the main reason and only reason for a forum like this.
Look at Sumo and see what you can find!