It’s about being able to change models mid-task. For example, I want to be able to plan using Fable but implement the plan using Sonnet, and that won’t work if this is implemented.
Actually, that brings up a good reason they can't fix it. Fable falls back to Opus when the topic is too "unsafe". That behavior requires traces than can move between models!
Star Trek is a post-scarcity society, those problems don't exist there unless you're in the middle of a crisis and on emergency power.
LLMs briefly seemed like this too, after subscriptions made the SOTA models too cheap to meter, but before they walked back on that and introduced quotas...
For plan it's relatively easy, just make the plan the artifact. The point is to ingest knowledge with one model and use it in another, and that is not necessarily easily expressible in natural language.