I think Genn is being supremely selfish, albeit with an understandable reason. On one hand he is driven by the loss of his son to the Forsaken (Sylvanas in particular) but on the other he's also deferring the humiliation of defeat and Varian's death at the Broken Shore onto Sylvanas - because the Forsaken make for an easier victory than the conflict with the Legion he should otherwise be focused on. Like all great tragedies Sylvanas' actions in Gilneas have spawned what is essentially a copy of her in the person of Genn, just as she was filled with loss and rage at her death and defilement by Arthas so does Genn feel the loss of his son and his country.
I think, unless there are somehow reconciled to one-another at some future point, the logical conclusion of Genn and Sylvanas' arc could only be death at one another's hands - Sylvanas' throat in Genn's teeth and Genn's body riddled with far too many arrows to recover from.