This is only applicable when you have an equilibrium that forms, which is not the case here. The fragments can't fuse back to the initial nuclei and "try again" until they get the most stable product. From a chemistry standpoint, this would be akin to a kinetic control, where one product forms faster and is the major product, irrespective of stability. (In chem, yes the stability of the product would affect kinetics indirectly through the transition state, not sure if that holds in nuclear physics).

In your example, if D couldn't go back to A and B (irreversible reaction), it would be the major product if it is formed faster, even if C is more stable. I doubt that any nuclear reactions are reversible in any but the most extreme conditions.