Why hadn't it happened this year, or last? Nanite is great, but IMO doesnt change the equation that dramatically.
The cost for a studio to manually decimate game quality versions have been small enough that of the will had been there, we could have been doing it for years.
Having loosely followed this space, I think I might actually have some input!
First off, you need a VR-ready environment. Not many companies are making these, so the only people equipped to roll this out is Valve, Meta, and possibly Unreal/Epic. Valve already did this alongside Half Life Alyx; they released certain map portions so you could freely walk around them with friends without playing the game. Meta is getting there, but the combination of low-power hardware and headset attrition doesn't leave them with much of an audience. There's also Epic, but they don't really do much in this space yet either.
So, obviously a paradigm shift is needed. And frankly, I think Universal Scene Description will be that paradigm shift. TL-DR: Nvidia came up with a new standard for 3D environments that interoperates game technology with digital effects. In theory, you could take your favorite Andor or Rings of Power scenes, drag their file into another app, and just start walking around in-scene. You may end up adjusting a few things vis-a-vis scale, but once it reaches adoption I can see it being pretty easy to work with and popular.
Maybe 'next year' is a little close, but the stars have definitely aligned already.
WASM is neat but extremely finnecky at the moment. I've seen cool stuff done with it, but I'm not convinced it's funny production ready yet.
WebGPU is sorta in the same boat as WASM, if a little more realistic of a bet. People will want hardware-accelerated browser experiences soon, and WebGPU will probably do most of the lifting there.