I'm really having a hard time following you. Do you understand what a stochastic model is? You seem to be treating diachronic structure as a deterministic system. Do people actually claim that?
I understand that it's based on probabilities. And I guess in that there's no categorical distinction between "effective" and "ineffective", unless you have a metric for a specific purpose.
However, my main point is the following: we can't claim to "understand" the past (in the sense of "why") if we can't predict the future (with a reasonable level of probabilistic accuracy).
To rephrase:
We CAN make explanations such as: "if this happened [eg, great vowel shift], then it was probably this reason"
But we can't really make explanations such as: "this is what the language was like, and therefore the next logical step was this [eg, great vowel shift], even with some probability (where that probability allows us to, a fair amount of the time, make the right guess)"