Not necessarily. 70 isn't all that old! Since it's unlikely we'll be running any marathons at that age, it leaves plenty of time for writing!
My deduction would be a mix of writing hassles and considerations for the TV series. I can vouch first-hand for how absolutely horrible it is to try to finish a piece of writing. (The only thing I find more difficult than nailing an ending is finding a good starting point.
) But it would also make perfect sense for HBO to have contracted things so that Martin could not release the final novels until the show had been resolved on the screen. As
annoying as I would find that, it makes really good business sense.
Can't begin to list off how many absolutely brilliant stories I've read that feel like they fall apart at the end. Whether it feels rushed, preachy, off-beat, or (in the worst cases) like it's completely missing its own themes...it's possible for things to faceplant right at the end. From the perspective of a film / television producer (especially if I'm investing in an
unfinished work), it would be smart to ensure that people need to watch my production to get the ending. If it comes out in print first, and people don't universally
love it, that could seriously impact the ratings / turnout when the film version releases.