Rune Wrights are more common than anyone realizes
Because of their classified nature, it would be really easy for others to slip between the cracks. If Montgomery hadn't gotten up to all the trouble he had in the first book there's a good chance he'd just be a jump mage who was good at reading magic. Never the wiser. The aliens have an amplifier of the Milky Way.
And we will get to see it when humanity fights them. Damien is very distantly related to the first Mage King.
It says that the Eugenicists often used the most promising kids as 'breeding stock', but even the best mage might produce a kid that isn't magic, especially in those early days of human magic. A few rejects dumped into the population from the 'DMA' bloodline eventually manage to reactivate the gift. Roslin Chambers will be the first mage Kira raises to be her Hand.
