Anduin is too young and inexperienced and too much of a pacifist. They should just make him the king of Stormwind while Genn leads the Alliance. He has way more experience and he definitely has the strategic insight and lust for battle.
Sylvanas is now warchief of the Horde. If Genn is leader of the Alliance, given how much they hate each other, that allows for much more interesting stories than if Anduin leads.