While there may be legitimate reasons people wear either modest shorts or sandals to church, personal comfort is not a legitimate reason. If you would not wear shorts or sandals to a court appearance or a business meeting out of respect for the formality and seriousness of those occasions, then you should do the same for our Lord at Mass. In any case, I do encourage anyone who sees people wearing shorts or sandals to Mass to presume that they do so for legitimate reason even if that reason is not immediately discernible to onlookers.
