> Depriving the owner of their revenue is theft
That doesn't make any sense. You can't deprive someone of revenue, because (at least given a few assumptions of non-totalitarianism) nothing guarantees that a thing will produce revenue. Owners don't have a "right" to revenue, especially in a capitalist economy.
> ...just as much as deciding to occupy a seat on a train without paying is depriving the train operator of revenue and is theft, claiming that the seat is unused or that you doing marketing is bogus
No, the problem with taking an unused seat on a train is not that you are "depriving the train operator of revenue." The problem is that the seat is a physical good, and once you take it, the train operator no longer has it. You have taken the seat from them (not the "potential revenue") -- and thus prevented them from selling it to a paying customer, or doing whatever else they wanted to do with it.
To see this more clearly, note that it would be wrong for you to take a physical good from someone regardless of whether they planned to sell it or not. The moral problem with theft does not lie in depriving someone of revenue.
Software isn't like that. By obtaining a copy of a program, you're not taking anything from anyone in a way that prevents them from using it. In particular, if the software has an "owner," you're not preventing them from selling it.