I'm not advocating an all out student simulator; just something that wasn't almost completely divorced from being a student. The game never really feels like you're a student at Hogwarts (and certainly doesn't feel like you're a 15 year old). I also disagree about your sales catastrophe -- even though I personally wouldn't want an all-out student sim, I think one that was set in a Hogwarts as detailed as the one in the game would have sold 10+ million to all the HP fans. Probably not as many as this game sold, but still a massive success.
Hogwarts and Hogsmeade are incredible (and are the most iconic part of the setting) in the game, but they're barely used. There's a lot of neat stuff in/around the castle that you'll only see if you go exploring, which is fun, but it really feels like a waste to not make that a major part of the game.
I thought it was just kind of above average as a video game (the open world and combat is pretty repetitive/boring compared to other games), and also just kind of above average as an HP game. It has great moments here and there, but it wasn't one of the better games I played last year, and if you cut out Hogwarts/Hogsmeade it probably would have been one of my least favorite (not that it was bad, just kind of bland outside of the HP trappings). It is beautiful, though.
Huge swathes of the game feel like they could just be generic wizard simulator. It needed a better story and many more interesting characters to make the world feel alive.
I assume they'll make a Hogwarts Legacy 2 considering how successful it was, but I think it needs some significant writing and mechanical changes to actually be a great game. I also think they really need to go off forwards or backwards a decade so they can have a clean slate to work with. I struggle to see how they could make a quality sequel using the same character.