I think GRRM told them that Danny burns down King’s Landing and kills almost everyone in it...but left out that it was an accident.
In the books, Kings Landing’s still has caches of wildfire underground rigged to go off in a chain reaction to destroy the whole city if one cache is ignited. How much dragon fire do you think it would take to ignite one cache and set the whole thing off? I don’t think it would take much. And only Jaime and Brienne know about it.
I’ve always thought this detail would come back into play. And I like the idea of Jaime not really saving King’s Landing but only delaying the inevitable. All because he is too proud to explain his actions.
Also Danny would likely lose everyone fighting for her that enters King’s Landing when this happens. Talk about a mental stressor! In her quest for the iron throne, she gets a whole city full of civilians killed, destroys her own capital, fulfills the Mad King’s plan and causes the death of her own soldiers. That’s pretty damn bittersweet.