Pleshette was asked to replace Anne Bancroft on Broadway in "The Miracle Worker."
She did and something rare happened. The N.Y. reviewers all came back to see the show again.
Pleshette won rave reviews. A number of the reviewers pointed out that because she was younger and had a quality of "vulnerability," it made the show even more exciting and powerful.
Bancroft was so commanding that there was never a question that she would win out over her student. Pleshette gave her character other qualities.
So no, it isn't so that no one else could play Anne Sullivan effectively.

I remember reading in Patty Duke's book that Suzanne's long nails scratched her face when she had to get slapped by her in The Miracle Worker. But Suzanne refused to cut them.

She replaced Bancroft in another play also,though the replacement doesn't have to match the original performance. Plshette/Bancroft has similar qualities.
Patty Duke,while complaning about everybody in her life,didn't like Pleshette's long fingernails, and thought she didn't keep them short enough for the play.

Agreed. Bancroft was physically perfect for the role and was fantastic in the movie. Wish I had a time machine and could have seen her on stage.

Thanks for the info LarryLep. I saw the same thing on the IMDB site after I made my post. That is what I get for not paying attention!! I still have a hard time picturing Suzanne Pleshette as Anne Sullivan. I do not believe anyone could match Anne Bancroft's performance in this role. Thanks anyway.

She played Annie Sullivan on Broadway replacing Bancroft for several months. I read Patty Dukes bio a few years back and she had some great info on the entire rum of The Miracle Worker.