It's just, y'know, the reason this show's so popular in the first place is because Martin was and is such a great writer. The structure was all up to him, and the characters and what they decided to do and where it took them. Very consistent, made a lot of sense, made for fantastic drama.
If they go into full-on "thanks for everything GURM, it's our now" fan fiction mode, they're basically... they don't know the characters as well as Martin does to handle them without his guidance, and even if they did, they probably don't care that much about making these people act "truthfully." The technique they use for the show certainly seems less refined. As the seasons go by, things will make less and less sense, and characters will be doing weird things and they'll keep falling in that direction, and then suddenly it's not that great story anymore that got people hooked. Instead it's just an entity that has to exist because it's a huge television show and you have to keep doing things required by that structure. They'll create lesser material by a considerable margin.
I mean, I guess the audience is in for the long haul now, but you hope that long haul is worth the time watching it and won't cause the numbers to drop, and people to look back in the future and say, "that really fell apart in those last few years."