But even at 77 years of age, Yasuo Kobayashi's shikko and suwari practice is better then just about anyone else. Doesn't seem to have crippled him much if at all; so perhaps folks need to take a better look at how he was moving to better understand how to avoid the price of their knees. Somewhere on YouTube there is a video of him explaining a part of these exercises a few years ago, I'll see if I can locate it and post it here.
Of course - and then there are those people who smoke a pack a day and live to 100. I think that you have to take a look at what's happening in the general population. The fact that there are some exceptions to a practice that is generally harmful doesn't mean that the practice is safe.