But you cannot say that there were no witnesses. You simply don’t know that.
There is a tradition believed and taught in the Eastern Churches (including Eastern Catholic Churches) that all of the Apostle except for St. Thomas were miraculously transported to the bedside of the Theotokos, were present with her when she died, and buried her. Then, when St. Thomas arrived a few days later, they visited her grave and found it empty. While this tradition does not carry the same weight as Holy Scripture, for you simply to dismiss it as unlikely because you think St. John would have mentioned it in Holy Scripture seems to me to be rather insensitive towards the traditions of Eastern Catholics, with whom you are in communion.