No problem making the large sums of money...I'd do the same, but come on, don't deprive readers of this for 7 years...Many of these fans have expressed concern that Martin will leave his fantasy series unfinished, in the same fashion as deceased author Robert Jordan did with his lengthy Wheel of Time series.
Martin is also strongly opposed to any type of fan fiction, believing it to be copyright infringement and a bad exercise for all aspiring writers. He never gives permission for any of his intellectual property to be used in fan fiction.
Also, he has stated he will take the series to the ground with him if he passed away during this time. So like someone else who picked the Jordan series, that wont happen with this...GRRM hates that and the series will end with him.
He is 62 though and surely he can live a longer and greater life, but he has had some major health problems in the past.