My understanding of the law is that a reasonable accommodation must be made when requested, not that everything must always be ADA compliant for all disabilities. One may conceive of different disabilities for which two reasonable accommodations might be mutually exclusive, so requiring two or more mutually exclusive accommodations to always be present is by definition unreasonable. Of course, it isn't impossible or unreasonable to caption a video; but it might be so expensive to caption all videos as to have the result that some just won't bother to post them. That would be unfortunate.