Here's your evidence: I would have bought House on DVD 15 years ago if there hadn't been the option to stream it illegally.
You might object this evidence by telling me that you bought all seasons of House only because you had been streaming it illegally before, and that you wouldn't have done so without previously streaming it – but in most jurisdictions, this kind of "business procurement" does not cancel out the harm done in the first case.
Anyways, the burden to disprove the harm done through me not buying it is on you.
I think what they are saying is there is no way to compare a good when it's free to when there is even a nominal cost.
My "counter evidence" to your example could be something like: I bought House on DVD 10 years ago because my friend who had pirated it told me it was a good show to checkout.
There is no harm done in the first case. You say you would have bought House if you couldn't have streamed it, but I say that's nonsense. If you didn't want to pay for it you would have borrowed it from a library or a friend.
You might object this evidence by telling me that you bought all seasons of House only because you had been streaming it illegally before, and that you wouldn't have done so without previously streaming it – but in most jurisdictions, this kind of "business procurement" does not cancel out the harm done in the first case.
Anyways, the burden to disprove the harm done through me not buying it is on you.