My point of Titanic 3D was Cameron knew how to convert it to great 3D because he is a masterclass film maker of directing/producing. Apparently George Lucas did not have much luck trying to do the same with Star Wars Episode 1.
Additionally Titanic 2D was a brilliantly well made film in its own right. The VFX of Titanic hold up supremely better than the feige films, nearly 25 years on.
Lastly 3D is not a gimmick. The films that came after avatar and tried to do the money avatar did but without the heavy lifting Cameron put in the film making was the real gimmick.
What Cameron has achieved with 3 point turning films in cinema (Terminator 1-2, Titanic and Avatar) has been far more impactful to the world of film making than all the 27 MCU films Feige has produced. Feige movies are popcorn fun produced by a well oiled cooperate machine as Disney. Cameroon's movies are personal art.