Imagine this version of Season 8:
Jaime catches up with Dany’s army about a day out of King’s Landing and informs her that Cersei has no intent of helping defeat the Others. Dany decides that she doesn’t want to deal with a Lannister army at full strength after she has been weakened by the army of the dead. She knows that the Wall has already fallen, so she doesn’t have any time to spare, and that none of Tyrion or Varys’ clever plans to invade with minimal loss of life have succeeded, so she decides to take the brutal action of simply burning down King’s Landing, so that there will be nothing left to resist her should she win. Cue disloyalty from Tyrion/Varys, but for now the zombie apocalypse seems more pressing.
Dany and company arrive in Winterfell. Now the Northerners’ distrust of Dany is more warranted, since they probably don’t have the whole story, and have only heard that she massacred the whole of King’s Landing. We can have our one last night before the battle, where Dany learns about Jon’s parentage, but the army of the dead are coming in, so no time for that. The climactic battle happens, which is now the final big event of the whole series, so things like killing off all the Dothraki and maybe a dragon won’t have consequences for later events. Arya stabs the Night King, and it’s all over, but now Dany has a dragon along with enough Unsullied to take over at least a sizable chunk of the now war-ravaged Westeros. To make matters worse, she’s not terribly pleased with what seems like Jon’s friends plotting to make him king. These friends do in fact convince Jon that he needs to kill Dany, so he does, and so Westeros takes the logical step of becoming an elective monarchy with an independent North.
I think this series of events makes Dany blowing up King’s Landing make much more sense, since even without additional justification through dialogue (which they should have definitely added!), it’s much easier to justify Dany blowing up the city when she still has all of humanity to save and is kind of on a tight schedule. Further, it makes the chilly reception she receives up North more justified, and it allows you to make an incredibly destructive final battle without introducing continuity errors.