I don't think the problem is shikko but sensei's attitude. I have seen people with knee problems encouraged to do shikko to the extent of progressive deterioration of their ability to walk properly.
I believe it is important for a sensei to encourage people to perform up to (maybe a little beyond) their abilities, but to know their physical limitations (we have students from teens to 60s). It should be acceptable for people to say they can't do something rather than be damaged (it is also up to the sensei to tell them they are underestimating their abilities!).
Happy New Year to all aikidoka!