Gratefully, our Japanese sensei declared years ago he had seen too many elderly Aikidoka stop practicing because of knee problems. He no longer requires seisa (except for brief formalities) and no longer teaches shikko and do suwari waza.
I'm so glad he's seen the light because I wrecked my knees at 20 bicyling with too much weight.
This is very interesting to know! Who is the Japanese sensei you are referring to? It would be good to have that data point.