The sound would survive and the name would not. A c before a is hard in English, so mac already says mak and the k adds nothing. More to the point, Mac is a fixed unit in Scottish and Irish surnames, and swapping in a k breaks the name apart. There is a second man in this story worth keeping straight: John Loudon McAdam, who gave his name to macadam road surfacing, spelled his Mac as Mc. Different man, different spelling, no nuts.