I don't think we're being pessimistic, just realistic. GRRM is now a rich megastar who spends huge chunks of his time travelling the world, attending press events and such, reaping the rewards of his fame. This makes it hard to get much writing done, for a man who can only write at home. When the TV show ends the series, I think he'll be even less motivated to write.
I love GRRM, he's one of my favourite authors, and I think he was set to match Tolkien's quality after the success of his first 3 ASOIAF books. I do think though that he's slowly let his legacy slide further and further away from him since the year 2000.