The reason why the doctrine of the closed door in 1935 (there was one in 1881 too) was necessary in the eyes of the WTS admin was that partakers reached 95,000 plus in 1925. It did drop to 25,000 the next year due to the disappointment of the end not coming in 1925. But they knew it could happen again.
They don't factor in how many partook in the first century and secular historians have said it is hard to calculate how many there were (or real ones per the WTS).
I never thought they would adjust the generation doctrine but they did and the rank and file outwardly ate the shit sandwich they were served.