The problem with the one technique scenario that nobody seems to have mentioned is the development of form, balance, control, movement and harmony.
The idea of learning only one technique might make you good at that technique but there would be no, or at least minimal development of these things. Nor would a student have any possibility to learn for themselves interlinking techniques as you would for the first year have nothing to link to.
Personally I like approach of x techniques for a grade and developing and adding to them as you go along( a comprimise between learning everything and learning only one technique).
Any way no matter what the techniques every increase in grade should demonstrate a clear (but not necessarily huge) improvement in form, balance posture etc, something that i don't think could be done on a one technique a year basis.
Just my pennyworth.