What do you think would have happened if all the Targaryens died at Summerhall?

From what I can tell the Baratheons would take the Throne, Steffon Baratheon having a Targaryen mother.

So that means Renly, Stannis and Robert would all grow up to be raised as Princes, Stannis probably be given Storm's End and Renly Dragonstone.

And if Steffon still decided to foster Robert with Jon Arryn Rickard is 100% agreeing to that betrothal so Lyanna is queen and his future Grandson King one day, no matter how much Lyanna throws a fit over it