D3 sold like 30 million copies. It's one of Blizzards biggest successes, ever. Is it still as popular today as 10 years ago? No. Why would it be, though?
I just reinstalled because talking about D4 made me nostalgic. But D3 was a massive financial success. If it "died", it was after a full life and a burial in a golden coffin, from which it rises seasonally for whatever audience remains.