Is it very plausible that she was buried and that's it?
No. It is not plausible that she was just buried and forgotten, because that would be ABSOLUTELY ATYPICAL behavior on the part of the early Christians. I don’t know how many of the bodies of the apostles were preserved and venerated, but relics of Peter, Paul, and at least several apostles are preserved. In St. Peter’s Basilica, the bones of about 15,000 martyrs are inside the columns of the Bernini baldacchino—those four twisty columns around the main altar. The tombs of who-knows-how-many other martyrs and saints are all over Rome and cities all over the Mediterranean region.
Whether or not all of the “relics” are genuine is beside the point. Nobody, absolutely nobody ever tried to claim that he had relics of Mary’s body. There is only one thing that could possibly have prevented somebody from falsely claiming to have relics of Mary: a consensus of all Christians that nobody COULD have relics of Mary.