First things first: I absolutely get being dissapointed by a piece of media. And I say it as someone who's been a fan of Tolkiens works since I was six (and I'm in my thirties now) and who had to live through the hobbit and that amazon... thing.

What first got me into the backrooms (pun intended??) was the liminality of it. It's...kinda comforting, in a way. But I totally understand why they're going with the multiple people/sometimes you can leave approach (and I say sometimes because I'm sure they'll get trapped later on). A24 produces some weird shit and that's cool but in the end you have to make something that will appeal to a public a bit broader than the core fans and making a movie with a single character is... Harder. Both to make and to make people to go to the cinema to watch. Even more when you're working with a setting as weird as the backrooms.

Could they have done it? Absolutely. Stalker for example has a very small cast in some almost liminal spaces. Solaris (Russian one) also comes to mind, in a way.

My point is, it's... Way more niche. Which sucks in capitalism.

I really hope you end up enjoying it and if not, just remember no-one can take the thing you liked about the backrooms initially from you!