The problem for me is that they calculated the story and miscalculated the necessary plot developments and world building.
What's left after season 6 could be indeed just 13 episodes of story - Dany reaches Westeros, she loses some battle, she gets convinced to get a truce with Cersei for the wight show and tell, they decide to face the White Walkers who invade Westeros and get defeated, and then it's the final showdown Jon / Dany vs. Cersei.
But the necessary character developments to make all those plot points happened left a lot to be desired, and felt rushed. Some more episodes and some more dialogue, plotting, planning and discussing would have done wonders to justify better the actions of the characters, or to present more clearly some pivotal moments, rather than having just the shocks for "OMG LOOK WHAT HAPPENED" reactions, nevermind that Arya flies in the air or that Euron has invisible ships.
For example - I get Dany going crazy, I don't like it and don't want it but I accept it and maybe that was George Martin's plan all along, but even if the writing was subtly on the wall, having it all happed in one episode is way too fast. Yeah, she showed signs of snapping but in one episode she feels isolated at Winterfell AND loses the dragon in a way that makes no sense AND loses her friend, advisor and confidant. This could have happened over the course of 2-3 episodes.