While it's generally not a good idea to outright ignore your fans, it is important that creators not fall into the trap of simply giving them what they think they want. Listening to fans' demands rather than doing what you want to do is a slippery slope, especially in the internet age when everyone has feelings of entitlement and can easily make the whole world hear their grievances.
It's hard not to applaud his response. Apart from anything else, outright telling him to finish the books before he dies is almost unbelievably rude - to be sure, he's a big guy, but having met him in person he gives every impression of being in very robust health. It's not uncommon to find people on forums talking about how fat he is and how he's probably going to die soon, and it's seriously unpleasant. If you think the fact that you might not get to finish the story is more important than someone dying - well, there's a problem there, yes?
It's the entitlement that's most bothersome, though. You don't get to claim some level of ownership of a series just because you like it a lot, and the author doesn't owe you anything. If you've bought a book, the contract is complete and the author is under no obligation to provide you with another. To be sure, it would be a real shame if Martin didn't finish his series for whatever reason, but he should finish it because he wants to, not because he feels beholden to his audience.
All we know about when The Winds of Winter will come out is "not in 2014," according to Martin's publisher, and analysts have projected release dates anywhere from 2015 to 2017. For whatever it's worth, Martin himself expressed optimism on his blog that this book won't take as long as the last one, because the fourth and fifth books were plagued by problems and took a long time to get working. At one point, he threw out a year and a half's worth of work because he wasn't satisfied with the direction it was taking.
Has A Song of Ice and Fire dipped in quality for its last two entries? Absolutely. But they're still not bad novels, they just weren't up to the standard we'd come to expect from the first three. Everyone just needs to be patient and let him finish the series on his own terms and in his own time. Wouldn't you rather he took his time to make sure he was happy with the book rather than rushing it?
Neil Gaiman put it best when the demand for Martin to finish the fifth book, A Dance with Dragons, was at its highest: "George R. R. Martin is not your bitch." We'd all do well to remember that.