She's poisoned her own legacy by what she did. There are enough survivors in the rubble that the story of the city surrendering and Dany burning it to the ground anyway, soldiers, common folk, innocent people with their children alike, no mercy for the innocent, will get out. How can she pretend to be the benevolent breaker of chains now? She can't. The people are going to hate her just like they hated her father. What Jaime killed him for planning to do, Dany just did.
Jon will have to do something now. Arya is going to ride back to Winterfell and tell Sansa what happened (confirming her worst fears about Daenerys). Then the North will declare itself independent. If Jon doesn't renounce her after the massacre at King's Landing, he'll be forced to turn against her when she starts planning a campaign against his homeland.