I think the future of three ruckmen is in danger because in the future they will be looking for 3 or more ruckmen! The difference will be the type of ruckmen that will be playing. In the future I would expect far more athletic, multi-dimensional rucks who rotate through a variety of positions and play a mutitude of roles. Vickery may be a prime example of this, touted as a quick, skillful forward/ruck.
I think you may find a ruckman in the middle, in the forwardline/backline and one on the bench.
Interesting to speculate regardless
Stripes