Can you explain how piracy is stealing?
Also, I think that you misunderstand the GPL terms. The aggrieved party in this hypothetical case is not the original author, it is the buyer of this modified software, whose rights to four essential freedoms [1] would be violated. These violations have a very real material cost.
BTw, it is common for people to believe that GPL requires publishing source codes on the Internet. It is not so. The requirement is to provide the user of a program source codes, so he can exercise the guaranteed freedoms. You can do it in any suitable form, publishing code is just the most convenient way to do it, but not mandatory.
Usually theft means that the original owner is deprived of goods - they can't use them, can't sell them, they simply don't the the original item. This is simply not the case for digital goods. HBO won't lose money when I download GOT, won't they?
It is disingenuous to say piracy doesn’t cost anything to the content creators.
However, when a creator goes out of their way to alienate me as a customer - by either dropping support for my platform, or by region-blocking - I happily put on my pirate hat and get what I want for free. There is no other viable choice for me.
I have bought a great deal of games I played in 1990s (pirated, of course - there simply did not exist a way to buy games legally in Russia at that time), just because I liked them back then.
Also, please stop calling copyright infringement 'piracy'. Piracy is a dangerous penal crime involving violence and an open theft of possessions. It has nothing to do with copying information.