In theory the 2.5 hour runtime might seem like a deterrent yet almost all of the highest grossing movies of all time — both Avatars, Titanic, Avengers Endgame — are more than 3 hours long. This isn't just a recent trend. If you adjust for inflation and look at the 10 highest grossing movies of all time, movies like Gone with the Wind, the Sound of Music, the Ten Commandments and Dr. Zhivago are all 3-4 hours.
If Wicked is a good movie, the runtime won't be a problem. When people are having a great time, they don't want a movie to end. It's only when the movie is mediocre that audiences really begin to feel the length.