Potter, like so many things nowadays, was always a distillation of many multiple elements, the redeeming nature of JK's books was that she picked her sources quite well, hid all the borrowing nicely, and wrapped it up in enough new flash and sparkle so as to wow enough people at the right time and create something that, deservedly, made its mark. None of the Potters were particularly well written, but they captured a moment, spoke to people in the right way at the right time, and that’s how (genuine) successes are made.
So, then, how to keep that going? Well the two ways are either prequels or sequels, as we know. And the prequels were already in the pipeline with the Beasts series — not instigated by her but by property sharing Warners, who wanted more films. So when someone comes to you with a shed-full of moolah and says "we want to do Potter on stage, any ideas?", and she’s already knee-deep in a prequel, then obviously the "easy way out" is to do "the next chapter" of what she'd already written at the end of the last book.
But…that was a story self-contained and completed within itself. Otherwise, one just keeps on writing Harry Potter, Son Of Potter, Harry Potter's Grandkids Go On A Magical Adventure, My Great-Grandaddy Was Harry Potter, etc, forever and ever and the whole thing just becomes a soap opera as opposed to the initial seven-and-done series she outlined way back when, and had previously completed as intended.
So *anything* coming after this, whether it be from JK directly or via another collaborator, such as Steve Kloves on the Beasts films, is a bolt-on. Like anything after Return Of The Jedi (and, arguably, even after the first Star Wars '77), it’s all about marketing and making money. Rowling's outline and story was told after seven books and eight films. That’s why that coda is there in both: to set up but not laboriously have to pay off the promise that, yes, life goes on past the end of the story.
But…in all likelihood, nothing actually happens in that story, once Harry sends Albus off on the train. The threat has gone. Life is normal. There is no story left.
So, desperation sets in. They need to come up with something, because someone wants to sink loads into a stage play. So with the prequel route already being mapped out, your options are rather limited to either either rehashing and repeating what came before, or trying to spice up what comes next, the problem being that what comes next, in "real" terms, is pretty boring and non-eventful.
It’s telling that the story opens with the same scene as closes the books/films, as clearly JK has no other story to fall back on. She's told her lot on this, so the obvious thing is to just go on with Son Of Potter but, somewhere along the line, in attempting to escape the rehash, it has become a *literal* rehash, in going back to the original books, which probably felt fresh and new and amazing at the time.
But then she relinquished authorship to her collaborators in the play, and this is maybe where her voice has been lost and the perception of "fan fiction" has crept in, since other voices, possibly treading too carefully, possibly trying to make their own mark, have somewhat taken over. And, as we know, everyone has their own take on where things "should" go, or how cool it would be to do this or that, and then get so confused with the backstory that the new version fails to notice it could all actually be quite simply fixed without further going down the rabbit hole.
But then you’d have no story, so in the desperation of wanting it to be totally different and yet exactly the same, you end up with a "watered down" endeavour of sorts.
But, then, it was never really going to be anything else. Was it?
