I meant that if I were designing a general-purpose retain-mode GUI library or 3d engine from the ground up and wanted to incorporate his principles as much as possible, how could I do that? Maybe a retain-mode approach is just inherently (too?) complex?

Correct me if I'm wrong, but I think your answer is in reference to using such a library, and I can certainly see how my question implied that, so sorry for the confusion if this is the case. Thanks for your answer regardless.

At that level it's similar tradeoffs. Consider what the code would look like if it were purely functional. In fact a good answer to your question would be for a thought exercise take a look at how XMonad is implemented in Haskell. That would be a completely different approach to the large, heavily coupled messes that OOP can sometimes lead to when modeling the state as mutable object members.

Thank you, I will look into that. Sorry, forgot to upvote you...fixed!

This is something I've been experimenting with. My intuition is that the scene graph will look a lot more like an AST made from algebraic data structures than an OOP actors network. Down that road, the system looks like an optimizing compiler with the really tricky added bit of iterating in response to user input.