I didn't think the quality suffered. I think a factor in the rising discontent was that hardcore fans were becoming increasingly invested in fan theories, some of which were pretty wild. With so many mutually conflicting fan theories bouncing around the forums, lots of fans were bound to be disappointed, whatever direction the show took. And there was all that time between seasons 7 and 8 for the fannish theorizing to reach critical mass. (Even I was rather disappointed that Tyrion wasn't revealed as a secret Targaryen.) And, of course, the show-runners HAD to proceed without the source material - can you imagine the world waiting another ten years for Season 8?