I already had a feeling we were essentially arguing the same thing...

Yes; the managed time approach in Glitch is very interesting and indeed a competitor towards addressing the same problem I originally raised.

The core of my original statement/problem definition was that real mutable state, i.e. throwing away history by default, was no longer the smart thing to do given current hardware.

From what I can tell you would also agree with that...

I would, but it doesn't necessarily have to happen with persistent data structures, and I would also argue that they aren't really sufficient (the history has to be useful, after all). They are convenient, and I'm using trees in my current implementation.