They rushed it because they wanted out,had nothing to do with the source material.
They'd probably have done a better job if they'd had more source material to work with. There's stuff from Winds of Winter, previewed by GRRM but not yet published as part of a whole novel, which is in GoT. Those were some of the best stuff in the later seasons.
The writers of GoT are very good at adapting. They're no good at creating. They couldn't exactly wait for more material to adapt, as GRRM hasn't released any new material in the story in ages. Unlike the characters in ASoIAF, actors don't age according to the text. Either the GoT story is finished while the actors are still viable as the characters in the story, or the story is left unfinished because the series writers have run out of material. The writers chose the first, which is commercially the only choice. And they were creatively crap, but that's how it goes.
