Good read. The big elephant in the room though: you likely won't purely hand-code the Stable version for much longer. So where's that split? Prototype vs. Prod? Feature Flags? Canary? 2 codebase nightmare? All of this already exists.

The message that hits for me is that of AI being a destabilizer while simultaneously being an accelerator. The Speed/Scale suggestion won't address this. A codebase no one understands, growing at machine speed won't go away just because you drew a box around it. The fix is likely more mundane stuff like process and role shifts, smaller PRs, tests, tooling, ownership principles.