Maybe trying to engineer addiction is what should be illegal, and if you want to question "how do you define whether something is addictive" you don't need an objective measure: you determine whether it seems like the people making the product seem to think that's their goal.
"you know it when you see it"
I guess I worded that poorly; it isn't merely that we don't need an objective measure: we literally don't need a measure at all, as the crime would be attempting to cause it, whether or not it was even possible to do, and so we simply do not care if the activity was addictive. If you are going out of your way to exploit the psychology or physiology of other humans in an attempt to use that to sell your product, maybe that is what should be illegal.
This would then mean that "our expert witness has strong evidence that my client's product area is not 'addictive', so my client could not ever be said to be engineering addiction" would not be a defense any more than "the plan my client came up with to kill their alleged victim could not possibly have worked, so my client can not be charged with attempted murder" is (at least generally, afaik) not a defense.