As you say, acts are a construct, and this point can be extended a little: the notion of 3 acts is itself a construct. One can say that a piece of string has a beginning, middle, and end, but what does that tell you about the piece of string?

I also agree that the reformulation you discuss here fits the character-growth story quite well. But even though character-growth stories are popular (especially with screenwriting gurus who use the hero’s journey gestalt), they don’t account for all or maybe even most filmed screen stories. So the mistaken belief-extraordinary journey-new way of acting formula, while accurate in characterizing many stories, doesn’t get at the essence of how a script / movie works.

I think the basic problem of plot / character oriented scenarios is that they look at the protagonist’s journey as center of the moviegoing experience. But it’s not. Characters themselves are constructs. When we talk about the 3 act structure as fitting the protagonist’s journey, we’re discussing how one construct serves another construct. None of it is real.

The question isn’t how or why the 3-act structure fits the protagonist’s journey but how or why it propels the audience through their viewing (or reading) journey. The audience is real.