Is there are term for when you believe that because something hasn't happened, it won't or can't happen?
This question is arguably better placed in the philosophy forum, but the answer is "false generalization" or "false analogy" meaning an argument that relies on superficial similarities to draw unjustified conclusions.
However, it's worth noting in the case of this particular argument, it's not clear that mechanizing farms did not in fact lead to loss of jobs, since there is currently widespread unemployment and a far lower percentage of people are now involved directly in agriculture.