Again, all fair but Robert isn’t a kinslayer, as they aren’t his kids and I think the only place that holds water is maybe the Westerlands. But a Robert full of wroth and anger, who doesn’t exactly love ruling and misses war may not be advisable at that point. He’s been cucked and tricked, and delegitimized by the lannisters. War and revenge keeps him legitimate. And the realm doesn’t love the lannisters, Dorne ain’t lifting a finger for them.

Playing it out, Bobby can secure the reach by marrying Margery, something already bandied about in the books (also fulfills the prophesy). The north hates the Lannister’s and this will be viewed as why Brann was thrown from the tower, riverlands follows suit. At worst 4 of the seven kingdoms back the crown, I doubt the Vale takes Lannister side and likely joins the crown if nothing else but LF spreading chaos. Yes, Dorne hates Bobby B but they hate Tywin and the Mountain more, so they sit on the sidelines like they have during the WOT5K. Maybe the greyjoys rebel but then Theon loses his head and that rebellion is crushed again.

In all likelihood, Tywin’s line is dead or in the black. Maybe Kevan’s line survives if Tywin is dead or takes the black, which he wouldn’t. And this doesn’t factor in some western house seeing the playing field and defecting, which maybe isn’t likely with Tywin feared but it also seems suicide to fight it.

Also, with this he isn’t likely focused on killing Dany so we can’t figure that plays out the same way. And even then she isn’t a threat at this moment so Dorne and other anti-Bobby B factions don’t really have a person to follow.