I agree that it's sad that GRRM failed to finish Winds & Spring- otherwise the GOT show may have ended up being great, and ended well. Then all the spin-offs would have a stronger foundation to work off of.
My personal reading of GRRM's work indicated to me that he was losing his grip by Feast & Dance (books 4 and 5). The first 3 are masterpieces that are responsible for the success of GOT but he seems to have run out of great ideas by Feast and was starting to repeat himself frequently. The spin-offs and extended universe are simply nowhere as great as the first 3 books and the first couple seasons of the show. They're derivative.
GRRM's decision to not finish the books is quite bizarre to the point where he is looking like a conman. It's almost as if he was to finish the series, it would reveal his true cards (a bad hand), jeopardize his shows (or interest in it) or something. If HOD declines in S3 and the other GOT spin-off projects don't perform his mega franchise would end up in a sad state.
The books definitely do not need decades to write; Sapkowski in the same period wrote and published 2 witcher books, and 3 acclaimed books of historical fiction. He wrote the last book in 2 years. GRRM also does not appear to show any signs of dementia either.