I agree that 1914 is the foundation that they built their authority on. And in the internet age, making any change to it would spread like wildfire among the exJW community. But I think we're underestimating their influence and control.
It's almost 110 years since The End officially Drew Near. 110 years might be the blink of an eye to Jehovah, but it's a long time for people. There's a reason they have always focused on how close the end is- urgency spurs people to action. If Rutherford had given a speech titled "The Great-Grandchildren of Millions Now Living Might Never Die", the organization might already be gone.
How many JWs know about 1925? How many would believe you, if you told them about it? How many, learning what really happened, would care? Heck, how many JWs still believe the BS narrative from the GB regarding 1975? How many are still in after watching the embarrassing "overlapping generations" video?? They don't need a particularly good explanation for how dropping 1914 still makes them God's Legitimate People. Just have Splane or Lett vomit up some fresh new word salad and the rank and file will shrug their shoulders and soldier on.