[dsfyu404ed]

The point of heir by birth is to eliminate questions of who will be heir and the power struggles. You can raise the heir and any backups with that role in mind from day 1 so they're amenable to it and you also have time to figure out if they're idiots or not and can assemble other governance structures (e.g. delegate specific responsibility into more merit-based advisor roles to keep your heir from screwing it up) to insulate against that if needed. The problem is when you have a bunch of idiots in a row or when you have a bunch of good kings in a row (and all the surrounding official positions get full of idiots). Generally speaking, of course.