Of heirs and prophecies

I was wondering, does anyone else have a problem with the whole "heir of..." business? Godric Gryffindor and Salazar Slytherin lived over a thousand years ago. No way they've got only one heir each. If any of their descendants had more than one child, all their children would be "heirs of..." as well. And their children's children. And their children's children's children. Seeing as the wizarding population is rather small and as children of witches of wizards more often than not are witches and wizards, too, there must be hundreds of "heirs of..." running around, who, by blood, have the same claims to being the heir of Slytherin as Tom Riddle. In addition, Tom is half-blood, so there is no reason to assume that his lineage is that of long line of inbreeding Slytherin descendants.

I really hope that Rowling doesn't build her story around Harry's being the heir of Gryffindor.

I also consider the prophecy as a problematic plot device. Harry wasn't destined to be the one to kill Voldemort. He was programmed. Voldemort for his part was challenged to antagonise Harry. Had the prophecy not existed, Voldemort would have had no reason to single Harry out and to kill his parents. Harry only goes after Voldemort, because Voldemort tried to kill him first.

Of course, it is well possible that Harry would still have to fight Voldemort, even if the prophecy didn't exist. But it is its existence which makes the confrontation inevitable in the first place. Self-fulfilling, anyone?