Martin has had 10+/- years since HBO started filming GOT, and he knew pretty quickly that they were going to continue the series to a conclusion. I read that he gave the series writers his proposed ending and they carried on from there. I think everyone is pretty much on the ball regarding why the books will not finish–he got way too complex plotwise and either didn’t know how to get himself out of it, or he got bored with the whole thing. If he really did intend for the books to continue he’s had 10+/- years to be pounding the keys to his computer and he’s not done that.
I remember reading about a seminar he participated in with Stephen King. The questions got to how do you write a novel, or something to that effect. King came out with “I write every day for X-hours.” Martin was gobsmacked. You mean you write every single day? Somehow he simply could not get his mind around that in order to write a novel you had to sit down and write every day.
That sort of summed everything up for me.