Acting isn't a game of one-up-manship. It's about exploring a character.
Heath Ledger gave one interpretation of the Joker, someone else can and will do another. They will be compared, and people will have a preference. But that doesn't make the latter performance any lesser a performance.
Think of all the people who have played James Bond. They all have a distinct character, even though 'James Bond' hasn't actually developed over the last 40 years (with the exception of Daniel Craig). Why not allow that to happen to the Joker? We've already got two distinct Joker portrayals, keep em coming I say.
Some people prefer Sean Connery to Roger Moore, but they're both equally as worthy performances. Imagine, in 20 years time, people had preferences ranging between actor X, Y, Z, Heath Ledger and Jack Nicholson. That'd be awesome, as well as faithful to both the character and the craft which Heath Ledger was so ****ing good at.