Dany burned her way out of every problem, but every time she did it it was to liberate oppressed people from tyrants. They edited it to be heroic, to portray Dany as benevolent to the people. Every character that entered her service loved her. To have her turn into a bloodthirsty tyrant is completely out of left field, even if she's spent 8 seasons burning people, because the people she burned are portrayed as having deserved it.
They spent the first 3 seasons with Joffrey as the principal villain. Then the next 3 seasons with Ramsay as the principal villain. In the final 2 seasons, which are only 13 episodes, they have 3 principal villains in the Night King, Cersei, and finally Dany. It's ridiculously cramped. For it to come across as remotely satisfying, Dany needed to either be edited to be a little more ambiguous throughout the series, or to cause her turn to be far more tragic. If things truly were slipping away from her, or if Cersei was winning and painting her as a villain successfully, being forced to do things she really doesn't want to do, etc. Even things like having Jon turn on her, or even better, the Night King converting her into the Night Queen and having her raise Jorah, Greyworm, Missendei, the Unsullied, the Dothraki, and the Dragons as a massive and actually trained army of the undead. Imagine a scenario where Jon has to side with Cersei to stand against the undead army led by Dany to defend Kings Landing and the realms of men. Far more satisfying.
Ultimately D&D fell into a trap of wanting to please fans on Twitter. They wouldn't kill Arya if you paid them to. They wouldn't kill Brienne, or Tormund. They boxed themselves in to needing a huge trope filled epic fantasy conclusion but that was never what Game of Thrones was about. They attempted to make an unexpected happy ending filled with standard finale cliches. It's no wonder it sucked.
