The northern king by election is more Anglo Saxon in feel and doesn't seem to tally with the rest of westerns which has gone down the inheritance route.
I actually agree with this - as with the "Wars of the Roses" comparison (demonstrably a source of inspiration for GRRM). From an in-universe POV it all makes sense. See e.g. Sam's automatic assumption that Jon would now be heir. This makes complete sense in that a) Sam would, based on his own background, assume that legitimacy is based on inheritance and b) would, in a fight among several factions, have to declare some sort of preference. Having the #1 man of a faction be your BFF is certainly as good a reason as any.
What I don't get is: how come modern audiences seem to presume the inherent legitimacy of the Targs more often than not? Not only over other forms of government but also over other ruling houses whose own claim to rule essentially doesn't differ from that of the Targaryan line?