But still, I'm skeptical.
If it is doable, the best way to prove it is to actually do it.
If no one implements it, was it ever really doable?
Even if there is no technical reason, perhaps there is a social one?
Yeah, to a zeroth approximation that's my current main project (https://github.com/zahlman/paper). Of course, I'm just some rando with apparently serious issues convincing myself to put in regular unpaid work on it, but I can see in broad strokes how everything is going to work. (I'm not sure I would have thought about, for example, hard-linking files when installing them from cache, without uv existing.)