The "Heath Ledger's death" thing is overblown. And even if that did get butts in seats to see TDK, those people were likely impressed enough to stick through with the series to see its conclusion.
That's the big thing. Heath Ledger's death helped bring them in but there is no reason to assume now they'd suddenly refuse to see the sequel.
Do people really believe there are tons of people out there who went to see TDK out of curiousity due to the press around Ledger's death and really liked the movie yet then would refuse to go see the sequel even though they really liked the last one purely because no one died this time? It's silly.