It's not a flawed assumption. If someone has pirated some IP (to open it up beyond just music and movies) I know they haven't paid me.
It is only hypothetical that they may end up paying for the content (or other content) later. Also, a culture that accepts piracy as legitimate drives prices down. If everybody is getting their IP for free( illegitimately, but widespread) then why would a consumer pay money for IP? So creators are forced to lower their prices to lower the barrier from "free" to "paid for". Which drives creators out of the business, or they create crap because they can create many many crappy cheap items in the same time it took to create on good item, that they can't sell anymore - but which people are happy steal.
Whether I want to seed the market with "free" stuff, because I think that putting my stuff in front of an audience will generate more sales is a business decision for me to make, not the audience who wants to rationalize their piracy. If you want to debate whether I should put "free" stuff out there, I am happy to debate that (and, by the way - I do some "free" work for the reasons above"), but that is my decision - and I get to decide which of my work I will give away. It's is not up to the pirates to rationalize their actions.
It really boils down to whether someone has permission or not. If the creator has stated that they don't allow copying of their IP, then to copy it is stealing. The creator may be making a stupid decision.... but it is their decision to make and no one else's.