Yeah, I feel like that definition of theft would lead to some really awful legal precedents. If a family member has decided to buy some product, and I talk them out of it, I would be committing theft by that definition. At least in the US, that would run afoul of the First Amendment. The company can yell at me all they want or ban me from their services, but they could not legally get the police to arrest me for a bad review or for convincing a family member.

Of course, just like with piracy, it would not provable that the revenue was guaranteed in the first place. But companies would absolutely try and litigate over it anyway.

I suppose this discussion is unrelated to this thread, though.