It’s not entirely fixed. That’s why there’s 6 content phases. But that’s the precise limit for how far Classic can stretch before it becomes something else. The breaking point is post-phase 6 changes.
Even just going into TBC would be breaking what Classic currently is; something Blizzard seems to recognize as implied by the recently leaked survey which strongly suggests when they do Classic: TBC those that wish to remain in vanilla land will have some option to do so.