In my opinion, footwork is key in aikido, so if footwork is lacking/non-existent, then the technique is inadequate.... If you aren't able to perform the footwork, or suwari-waza due to bad knees, or not being limber enough, or whatever, then you aren't adequate enough to progress through gradings, simple as that.....
So you are telling me that if an older person joins aikido who has bad knees that they can't test and go up in rank? You can do the same techniques from standing as you can in suwariwaza. In my dojo, a person with bad knees wouldn't even be able to test and would be 6th kyu forever because suwariwaza shomenuchi ikkyo and suwariwaza ryotedori kokyuho is on the first test.
That just doesn't seem fair to me......