I think about this all the time.

A high-quality replica takes tremendous effort and time, but it’s not really financially viable.

I’d be happy to be proven wrong.

Oh, and I’d be super happy to pay for such a replica too! For example, $99 a year for a Mac OS X Jaguar to Tiger era skin/theme/desktop environment.

I think in the long term, such a project could be more sustainable than projects focused on an “original” experience.

KDE, GNOME, etc spend a lot of resources on new features and overhauls to enable new features. A fixed-feature replica project by comparison would eventually achieve a “finished” state, with resources instead going towards optimizations, bug fixes, code correctness, etc which are overall less demanding and make unavoidable tech migrations (e.g. X11 → Wayland) less of a pain since the project won’t be mired in tech debt.