I think that the whole point of the One Generation = Two Overlapping Groups was to buy time.
They didn't seem to need that doctrine but they still invented it.
So what's the expiry date?
I would suggest the 2030s. By that point Mark Sanderson will be in his 70s. He's been used as an example of the second group that overlaps with the first.
It wouldn't be a surprise to me if they're not already re-examining scriptures with the view to create a whole new end times dating system.
The WT needs the end to be 'close' to 'create' urgency. As 1914 fades into the distance it will be harder and harder to maintain that urgency.