Personally, based on what I know of the musical, it doesn't sound to me like it has a totally different agenda than the play on which it is based.
And it certainly would not exist without the play.
Flower Drum Song (the original version of the musical) billed itself as "Based on the novel by C. Y. Lee," even though it was an exceptionally free adaptation, truly more "suggested by" than "based on." But apart from contractual matters, it probably depends on your personal interpretation of what the words in question mean (and what they mean in context).
Guys and Dolls billed itself as "Based on a story and characters by Damon Runyon," but if you read the story in question, you'll see how the musical basically turns the story on its head, taking two character names and their professions and the idea of a crap game and almost nothing else. It's really only suggested by the story, along with the general milieu of Runyon, using character names (and a few character traits and professions associated with those characters).
My Fair Lady billed itself as "Adapted from Bernard Shaw's 'Pygmalion,'" which to me would suggest something freer than "Based on," but My Fair Lady sticks very closely to Pygmalion, carrying over vast chunks of dialogue from the play and borrowing other ideas from Shaw's published screenplay.
It may be that the Shaw estate thought that Shaw would have preferred "adapted from" given his famous refusal to allow any musicals to be based on any of his plays after his extreme displeasure with The Chocolate Soldier.
So I just don't think there's an answer. And, again, the producers may have no choice in the matter. Contracts with the Durrenmatt and Valency estates have already been signed.
Not to mention that there is already an opera based on the Durrenmatt play and another musical based on it that I believe is currently playing in Vienna.
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