I'm really not sure why they couldn't have saved the dragon getting killed for this episode. It was such a weak and boring death last episode.
Imagine if they had 2 dragons destroying the walls at the beginning, right up until the point where the Lannister army surrender, and then as they lay down their swords, the dragons land atop the walls. Cersei realising that her men have surrendered rages and orders Euron to kill the dragons, and he manages to ballista one of them to death. THEN Dany flips her lid and burns the whole city. It would have had so much more impact.
Don't get me wrong, i'm fully supportive of The Mad Queen and I think it's the best way to conclude the series, they just needed more time to flesh it out. It's become incredibly obvious that their story that they only had 6 episodes worth of story left was complete bullshit and that the series was cut due to budget constraints. The pacing has been so rushed compared to the previous 7 seasons.